Secure Minecraft?

Unfortunately, I don't know how to secure my worlds. I would like to go into multiplayer mode, but it's probably crashing, I don't know, I've saved my worlds.

The multiplayer is independent of the single player.

How do you secure worlds? You copy them from the corresponding folder into your own, which you can store on the desktop, for example. You can get to the worlds by entering "% appdata%" in the Windows search, pressing Enter and then looking in the .minecraft folder in "saves". Every single world has its own folder.

But are my worlds safe after a crash?

Yep