Can he still use that?

In minecraft java it is possible to copy your world from the .minecraft folder and copy it to a stick. I wanted to give the stick to my friend to see my world. I've already copied it from one device to the other using the stick. But what if I have mods active and place blocks that are not actually there in minecraft and then copy the world. My friend won't have any mods then. Will the blocks I placed using the mod be there?

No, he needs exactly the same mods. Without mods, he will not be able to enter the world with vanilla.

Yes. In / minecraft there's a folder called "saves" where all worlds are. And if you place blocks with mods and run the world without mods, the blocks are gone.

No, he needs the mods too. Otherwise he won't see the mod blocks / can't load the world.

What if I copy the mod file to the stick.

When he has all the mods he can enter the map.

It should only be a mod. And then I can copy the file to the stick, I think then it would work