The inertia, how sluggish the pedaling feels, is affected by flywheel weight/speed or incline out on the road. Cheap trainers on max resistance with a low gear simulate incline riding (“pedaling feels like quicksand”). Expensive trainers on low resistance and a high gear simulate flat road riding (“real life road feel”).
You get better at the type of riding you practice.
Which makes you a “climber”, the type who performs best at low inertia.
I’m using a Tacx wheel-on trainer, max resistance, with a power meter on my bike. It feels like pedaling in sand (steep hill). My outside riding is mostly on the flat. I don’t care much that my indoor training doesn’t feel like my outdoor riding - it’s cheap, quiet and I get fit.
climbing out of the saddle will output more power because body weight is contributing to power output. And out-of-saddle also reduces muscle fatigue, but often increases demand on aerobic system.