I’m certainly in the ‘bikes are for outside’ camp. Turbo trainers can be really effective ime but for me they’re a tool for fine-tuning fitness for something specific, they don’t replace riding outdoors. I’d rather be out in the damp and grey for a couple of hours than using a trainer.
I have an old trad trainer, a HRM, an over/under threshold session that get results if I stick at it once a week for 8-12 weeks and a playlist of techno or thrash metal that works. Simple stuff. But I need to be motivated by something in the first place otherwise my regular riding is enough to keep a reasonable form, I’d rather do hill reps or a hilly loop than an interval session and I reckon it’s almost as effective overall.