BSA file

A BSA file (or Bethesda Softworks Archive) is a type of archive data file used to store game assets in Gamebryo and Creation Engine games (with the BA2 format replacing it in Fallout 4 and Fallout 76).

Gameplay
Other game data such as item, character, and world space information are stored within separate ESM record files. BSA and BA2 files are found in the Data subfolder of any parent game directory (i.e. where the game's .exe file is found).

Individual files can be extracted from BSA and BA2 archives using third-party tools such as Bethesda Archive Extractor (B.A.E.).

Fallout 76
* - 00-prefixed archives are update content