I’ve done basically all workouts since November on rollers, having built up experience and then bought rollers with a magnetic resistance unit.
The advice above is good, I’d add that weirdly no handed is actually more stable than 1 handed, and will teach you how to steer with your hips.
It also definitely carries over - when I first learnt I got a bump in FTP, likely from a more efficient pedal stroke. I’d also like to think it makes you more stable outdoors, and haven’t fallen off so maybe that proves it!