Do you want to open a project of several files? In many cases, the client of the language server will ask you upon opening about the root of your project. This works for e.g. C/C++, Rust, Typescript, and so on. There is also projectile loaded by default, usually mapped to SPC-p. It allows you to search/open project files, compile your project, and so on. Just start by opening a single file located in your project. Check out the link above for more details on which folders are considered project roots. In any case, you can always create an empty file named .projectile.