This is the 4.01 version of PSP Legacy (as opposed to the R4 version on most PSP homebrew websites). The three folders go into PSP\GAME
4.01 was an update made by Six-Shoota in 2014 and contains additional bug fixes. This zip file contains PSP Legacy 4.01 bundled with the correct WAD files and music. legacy_alt contains alternative music. The first music pack is from Duke, the alternative pack is from Antonio Aiello (with Duke's music used for Heretic).
Hokuto no Doom MOD is a mod which turns Doom 1 into a Kung Fu game.
PSP Legacy is not a Doom source port. It's based on Quake's source code and has hardware acceleration. The config.cfg configuration file assumes you're using a jailbroken Vita / PSTV and are using Adrenaline with the right stick mapped onto the right hand buttons. It this doesn't suit you, delete the config.cfg file and set up your own configuration.
Apart from the bit of information in the readme file, the only other information about the 4.01 version is here:
You can get the original release files (with source code) here:
Live Music Archive
Librivox Free Audio
Metropolitan Museum
Cleveland Museum of Art
Internet Arcade
Console Living Room
Books to Borrow
Open Library
TV News
Understanding 9/11
