Hi, can I set up the spawn of all bosses so that they spawn rarely around the world and already in loaded chunks, for example as zombies, spiders and the like, otherwise it turns out that new players cannot find bosses who were killed by old players a long time ago?

How to do it?

Yep! You can do this with a separate mob spawn control mod like In Control: https://www.curseforge.com/minecraft/mc-mods/in-control

I'm sorry, but I can't understand how this mod works, is there any other way to implement it?

There may be other mods like it, but none that I know of.
It’s not that hard to use In-Control. It’s well documented and the people on the Discord server are helpful. Take the little bit of time to learn how to use it and you will be glad you did.