Tool slots accept items that have "acceptable_tools" tag. You can simply add it to any item you want by datapacks. 

Useful links:
https://minecraft.gamepedia.com/Data_Pack
https://minecraft.gamepedia.com/Tag
https://github.com/Tiviacz1337/Travelers-Backpack/tree/1.16/src/main/resources/data/travelersbackpack/tags/items

• Equip button is visible, when player holds backpack in a hand and doesn't wear any. If button is clicked, the backpack will be worn on the player.

• Unequip button appears, when player wear a backpack. To access this button you have to open inventory [default B]. If button is clicked, the backpack will be added to the player's inventory.

• Sleeping bag button is visible, when backpack is placed on the ground. If button is clicked and there's enough space, the sleeping bag eventually will be deployed. If sleeping bag is already deployed and player clicks the button, sleeping bag will be folded back to the backpack.

• Ability slider appears only if backpack has ability, if not slider will not be accessible. Allows player to choose if ability should be enabled or disabled.

 

• Sort button allows player to sort all sortable slots in the backpack's inventory. Shift clicking sort button enables unsortable slots selector (Blue sort button) and can be disabled by shift clicking again. Player can click on slots and mark them with blue color to make them unsortable and untransferable.

 

• Quick stack button detects all of the items in player's inventory that can be stacked with the same item inside the backpack, and places it in there.

• Transfer to backpack button allows to transfer all items from player's main inventory to the backpack's inventory. Shift clicking button will also transfer all of the items from player's hotbar.

• Transfer to player inventory button allows to transfer all of the items from backpack to player's inventory. Items from slots marked as unsortable will not be transferred.

• Inserting and extracting items from base backpack inventory - input is on the top, output is on the bottom of the backpack.

• Inserting and extracting items from crafting grid - input is on the rear side, output is on the front side of the backpack.

• Inserting and extracting fluid from fluid tanks:

 Left Tank - input and output is on the left side of the tank.
 Right Tank - input and output is on the right side of the tank.

Pipes, transporters and tanks used on screenshots come from Mekanism mod.

For first you have to wear backpack (Hose can only be used with backpack equiped), then you have to attach hose to tank, by pressing right mouse button. Tank is selected if its border is white, this also means that hose is active. Hose attachment resets everytime when you take off your backpack or remove hose from inventory, so make sure that any of the tanks is selected. There are 3 hose modes, which I'll describe below:

• Suck Mode

You can pick up the fluid from the world or - if clicked on suitable tank - transfer fluid from clicked tank to your backpack's tank.

• Spill Mode

You can spill the fluid into the world or - if clicked on suitable tank - transfer fluid from your backpack's tank to clicked tank.

• Drink Mode

Allows player to drink some tank's contents - not all fluids are drinkable, some of them may give special effects though.

Additional Tips

• Swap tank attached to hose

If you wear a backpack and hold hose, click N to swap tank which hose is attached to. (Configurable in controls)

• Change hose mode

If you hold hose, hold Z (Configurable in controls) and scroll up or down (can be set to use keybind only in config) to change active hose mode.

New abilities available only 1.16.5+

Netherite Backpack - +4 Armor
Diamond Backpack - +3 Armor
Gold Backpack - +2 Armor
Emerald Backpack - Gives some cool particles, nothing special
Iron Backpack - +2 Armor
Redstone Backpack - Redstone block in a form of a backpack. Provides strong redstone signal
Bookshelf Backpack - Holds mighty knowledge, place around enchanting table just like the bookshelf to get higher-level enchantments!
Sponge Backpack - Works like sponge block and stores water in backpack tanks. It is required to have empty or not fully water filled tanks for ability to work
Cake Backpack - Feeds you slowly with delicious pieces of cake
Cactus Backpack - Acts like real-life cacti, If it's raining and above backpack is clear sky, the backpack will fill empty tanks with water over time. If you want some more water, dip yourself! The backpack can fill with water while being a block too
Melon Backpack - Can drop up to 3 melon slices when harvested
Pumpkin Backpack - Staring at the enderman no longer makes him angry
Creeper Backpack - If player is going to die, creates explosion which damages every creature around, granting player absorption, regeneration and fire protection effects. Ability is available every few minutes
Dragon Backpack - Backpack for real warriors, combines Magma's Cube and Squid's backpack abilties, plus gives you strenght and regeneration! You'll be invincible!... but.. what did it cost?
Enderman Backpack - Extends range distance by 1 block
Blaze Backpack - Immunity to fall damage and small fireballs sent by blaze
Ghast Backpack - Ghasts see you as an allay, unless you attack them
Magma Cube Backpack - Immunity to fire damage
Spider Backpack - Have you ever tried climbing walls?
Wither Backpack - Immunity to wither effect
Bat Backpack - Provides seeing in the dark! Batman Batman Batman
Bee Backpack - Attacking creatures stings them causing poisoning, which lasts 4 seconds
Ocelot Backpack - If nearby hostile mob, grants speed effect
Cow Backpack - Cleanses all negative effects
Chicken Backpack - Have you ever wanted to be a chicken? No? Well, now you are! You'll lay eggs!
Squid Backpack - Grants water breathing and night vision effects while being in water

Old Abilities 1.12.2

Bat Backpack - Provides seeing in the dark! Batman Batman Batman
Bookshelf Backpack - Holds mighty knowledge, place around enchanting table just like the bookshelf to get higher-level enchantments!
Cactus Backpack - Acts like real-life cacti, If it's raining and above backpack is clear sky, the backpack will fill empty tanks with water over time. If you want some more water, dip yourself! The backpack can fill with water while being a block too.
Chicken Backpack - Have you ever wanted to be a chicken? No? Well, now you are! You'll lay eggs! 
Creeper Backpack - Sneak on another player to make them jump in confusion as they think one of those green monsters is behind him/her. You can do it every few minutes.
Dragon Backpack - Backpack for real warriors, combines Pigman's and Squid's backpack abilties, plus gives you strenght and regeneration! You'll be invincible!... but.. what did it cost?
Emerald Backpack - Gives some cool particles, nothing special.
Pig Backpack - You'll hate this sound, believe me!
Pigman Backpack - Provides fire resistance.
Rainbow Backpack - Now all people on the server will see you! Creates a trail of notes behind player, while sprinting. Also gives jump boost and speed - isn't it cool?
Slime Backpack - Has an incredibly useless "ability". Makes the player leave a slimy trail of particles whenever is running, and make that splishy splashy squishy sound on each step as well!
Squid Backpack - Grants water breathing effect while being in water.
Sunflower Backpack - Will feed you while you stay in the sun, slowly. At the very least you shouldn't starve.
Wolf Backpack - Checks around for any wolves that may lurk around. If any of them gets mad at you, it will promptly forget about the whole deal. Also if it's full moon, you'll gain werewolf strenght!