I’ve always found that the easiest first step is checking Google Photos’ Trash (it keeps deleted pics for 60 days). If they’re not there, what worked for me was using DiskDigger (no root required) or connecting your phone to a PC, enabling USB debugging, and running PhotoRec/Recuva on the internal storage or SD card. For SD cards I pull it out, pop it into a card reader, and run Recuva on my PC—yields the best results. Just stop using the device right away so you don’t overwrite the files.