This is a pretty faithful migration of my bedrock addon teleporters found here https://www.curseforge.com/minecraft-bedrock/addons/teleporters I migrated it to java as someone requested it.
Making a teleporter is easy. you need 4 things.
1 block of either obsidian, crying obsidian (bottom)
1 block of waxed copper
1 block of either quartz, amethyst or emerald (top block)
and a redstone wire on the very top
to active the portal power the redstone wire. A button on the side will do this nicely. The game checks ever 1.25 seconds to see if it needs to teleport (it checks the wires power at that time)
It will send all entities within 5 blocks to the FARTHEST portal with the same type. So quartz to quartz and amethyst to amethyst.and emerald to emerald. if you have more than 2 of 1 type, the farthest destination away from your current position gets selected. if you have 1... you don't really go anywhere.
You may have up to 6 teleporters. This is mainly due to a constraints of how minecraft works as each teleporter has a ticking area next to it to keep it active when a player isn't around.
When the teleporter comes online. It looks at the top and bottom block to understand what it sends and where it sends it to. Changing these after the teleporter is online has no effect. If the waxed copper block is destroyed the teleporter will go off line and no longer count as one of your 6 teleporters.
The teleporter can either send players items and mobs. or just players and items.
Obsidian - mobs, items, players
Crying obsidian - items, players
If a player doesn't want to be teleported they can carry a mundane potion in their hotbar on them and the teleporter will ignore them when it activates.
Because the teleporter activates via redstone signal you can setup all sorts of contraptions to activate it when you want (and not till then)
Notes on The java edition
It seems to work pretty well, the only hangup i know of, if you build 2 teleporters close to each other and destroy one it might break the other (make it so it cant be found when not in the area). This due to how java handles forced loaded areas. I might fix that in subsequent versions but for now, just to get it out there, i left this issue in place.
The crying obsidian here doesn't work quite like it does in bedrock. bedrock can group "mobs" (it's a family) and java doesnt have that. it would require me to make a big list of creatures to exclude, which seemed like overkill. Instead i just take items and players. the net difference is things like minecarts and item frames and such don't get teleported with crying obsidian. if there is strong interest i can fix this as well. it's just didnt seem necessary.
finally, this one runs a tiny bit faster/is more responsive than the bedrock one. the bedrock one is setup to basically check once every second. java had a hard time always having the redstone active with a button push in that time. sometimes it would work, sometimes not. so i made it run a bit faster in between checks (3/4 a second instead of a 1 second) to make sure a simple button would always work.