I have no clue how to select and move frames within a video layer (aside from splitting the layer at playhead and having then 2 layers but I still don't know how to even delete many frames at once to make it not painfully tedious). I'd be really grateful for your help!
After you split the layer at the playhead, hover your cursor over the end that you want to shorten. You'll see a different-looking cursor that looks like a bracket with arrows coming from the center pointing left and right. When you see that cursor, you can drag to trim the clip.