[KaktoR]

Experience, knowledge, repetitions.

I would say you can go with game engine repetitions most of the time.

I can tell you that nearly 99.9% of all unreal engine games using either zlib or oodle compression. Older engine versions like UE3 used lzo algo mainly back then.

Most of the unity engine games (if they have bundle files) using lz4hc compression with level 9.

RE_Engine (Resident Evil and all these games) were compressed with either zlib or zstd, but to be honest I didn't worked with this engine since years so things might have changed.

Precompression just means that already compressed streams will be decompressed, that's why the output will be larger than the input, so you can compress the decompressed streams with a better compression than the original used compression. Like if you compress some random folder on your pc with 7z. The compressed files (7z archive) will be smaller then the original input. If you extract the compressed 7z archive, the output will be larger again. That's how precompression works to keep it simple.

I think a and b are the same, or I am really confused.

To keep it simple, these steps in this order:
Precompress -> remove duplicated data -> apply stronger compression