There's a tool that attaches to your drill or impact driver. Kinda expensive, but when you NEED it, it's worth every penny.

"Oscillating cutters" are good for close cuts in tight spaces.

Oh, I should clarify, "the trades" isn't exclusively plumbing. I can sweat pipes and lay out a house but I'm no expert

That being said I always used a little stubby cutter for pipes close to walls

There's other designs, including ratcheting ones that look so much easier than what I used

Oscillating saw with a semicircle blade and a piece of paneling behind the pipe in case you get squirrelly with it