The PSP does not have analog "triggers" like modern controllers. It has two digital shoulder buttons on the top edge:
In many games (shooters, racers), the R button is mapped to "fire" or "accelerate," functioning like a trigger. But it's a simple clicky button, not pressure-sensitive.
If you need analog triggers, consider a PSP-to-PC adapter with a DualShock 4/5, or play on Vita/PS TV which have rear touchpad triggers.