If you want the actual GTA San Andreas experience on your phone—with the official Definitive Edition graphics—you don't actually need an emulator. Rockstar Games released the directly for Android and iOS (available via the Netflix Games catalog or as a standalone purchase). This version features better controls, lighting, and performance than any PSP mod can offer. Safety Warning
Often, these mods allow you to play as Carl Johnson within the Liberty City or Vice City maps.
If you decide to download a modded ISO from a community source, here is what you can typically expect:
Move the extracted .iso file to a folder on your phone (e.g., /Internal Storage/PSP/GAME).