Must all things be created by yourself if you sell a product?

For example, if someone makes a game, and he uses another people’s certain mesh, which the creator allowed, is this legal? Can you borrow other players makings, or do you have to make them all by yourself?

It is legal as long as the creator allows it, and it isn’t a copyrighted entity(i.e. someone can model Sonic the Hedgehog and make it free to use for any project, but that still doesnt give you the right to use it in any way since the character is owned by Sega.)

If you use other peoples work it is good to seek permission and give credit to them in your published game. Some places you can get free meshes, textures and they are open source. Which means they can be used in commercial or paid work unless specified. Some open source can be re-used for free but the author can still restrict from conmmercial projects. Always check the source and always try to get permission and always give credit where credit is due.

Ok, so I must ask the persons about it, ok. But what about if the Unreal 4 has created it? For example https://www.unrealengine./content/a9ca5e1f1c9a42b59d98bccbfff06f71 can you use those or do I have to make my own?

You can use those and everything else Epic gives us in your commercial or non-commercial projects as long as they are made using UE4. You can also find this kind of information in the description section of those assets.

ok thank you. can I ask one more thing, for I noticed there are also other sites where you can download 3D content. How can you do it? Do you have to use Blender or can you just download them just like that?

Depends on the format of those files since UE4 supports FBX and OBJ only. In any case though, you should pass the models you get through a modeling program first to make sure it has proper UV’s and all.