Let’s say I create a project in Unity and add some assets and packages to it. I then upload this project to Github (using Github LFS and Github’s default Unity gitignore). When I now clone this project on another PC and open it in a fresh Unity installation, will it work right away? In other words, will the Github project contain all the necessary files, packages, etc., or do some of them have to be installed on the 2nd PC manually beforehand? What about things like Cinemachine, TextMeshPro, etc.? Are they contained in the Github project?
Yes in general this should work.
in each Project there is a meta file that contains information on which unity packages are used in this project. Unity should download this automatically.
All other asset packs (for example from the asset store) are located in the Assets folder either way and are thus present on the new computer as well.
Only things that come to my head which will make problems are absolute file paths. Avoid this at all costs.