The way I understand it, you have a multiverse, made up of different universes like the ones Dr. Strange traveled to in Multiverse of Madness or the ones shown in What If.
Inside each universe, you have a timeline. If you alter that timeline, then you create a branch timeline (which this Loki was from a branch created by the Avengers in Endgame). If the timeline branches (Nexus event) then it creates its own universe with its own timeline (this avoids the grandfather paradox).
To make it more complex, you also have the various realms, which are basically places in those universes. Stuff like the mirror dimension, quantum realm (which according to Loki is adjacent to that respective universe) fall into this bucket.