So I wanted to start getting into casual speedrunning (mostly Pizza Tower,) and I have no clue where to start. My Steam Deck's my most powerful machine, so I'm wondering if it's possible to record runs on it, and how? I know basically nothing about recording equipment or software, so any help would be appreciated!

Hello there new runner!

The Steam Deck is very capable of streaming/recording runs. I sometimes use it myself. The easier path by far is going for desktop mode, install OBS & use it to do desktop recording (make sure not to reveal any sensitive stuff on videos/stream). There is a difficult path setting up OBS for game mode. I tried it, but couldn't get it to work to my liking yet.

You find a broad guide about starting with recording here: https://www.speedrun.com/forums/streaming_recording_equipment/qlnmy and resources on the help pages: https://www.speedrun.com/support/learn/resources-for-speedrunners