I wrote my self a note about this last year to demystify the internal data structure [0], if you want to check it out. But the TLDR straight to rabbit holes; is that commits are a directed acyclic graph (DAG) [1]. And the file tree snapshot is a fully persistent trie [2]. And the commit DAG points to different snapshots. Branches are name shortcuts to commits.

[0] https://sransara.com/notes/2019/build-yourself-a-git/ [1] https://en.wikipedia.org/wiki/Directed_acyclic_graph [2] https://en.wikipedia.org/wiki/Persistent_data_structure