It's mostly core and upper body work. I've neglected mine the past month and it shows after about 20 miles or so. My neck and shoulders feel tight. I have permanent neck, back and shoulder injuries from car wrecks and being hit by cars on my bike, so if I neglect my full body workouts those old aches and pains resume.
I don't go to a gym or use weights, other than water jugs at home. I rarely use stretch bands. Just body weight exercises and stretching. Even if I don't feel like doing 30-50 pushups, etc., I'll try to do 10 semi-pushups off the countertop and/or door frame, along with full range of motion exercise every day just to keep a little looser.
Most YouTube tutorials for physical therapy and basic exercises are way too long, taking 15-30 minutes to say what could be done in 3-5 minutes. Check the Bowflex channel for some good 2-3 minute exercises that require little or no equipment. They don't try to sell Bowflex gear on every video. They're succinct.