Interesting this might help explain it. https://sims4.crinrict.com/eng/2014/09/user-files-explained/
I use 7-zip for my CC and mods. https://www.7-zip.org/
You can have it open with different programs and seems like that was the last program that opened that type of file.
This is the package editor that modders and CC artists use to edit package files like that:
https://the-sims-4-package-editor.en.softonic.com/
Those are the default files for the game and the avatar cache one is new to help prevent the bug that was happening with CAS making the game crash. Think of caches files like internet cookies, they build up memory over time.
