Move from Subdomain to Root

The first part of your question hangs more on how much control over web server configuration you have. If you can point new domain at the same location in filesystem — then no, you won’t have to move a thing. Otherwise you do indeed need to move files.

On other hand it’s not that complicated to just move everything from one folder to another. If you have something like typical cPanel powered server it likely has file manager right on server.

Then you need to update details in database. The most important would be to use tool that can do so safely and is aware of serialized data (which WP uses extensively on storage). WP-CLI can do this (among other tools) and is useful to have around in general.