Apps I currently use:-

Zwift - as others have said there are a LOT of ways to use Zwift. Racing, group rides, workouts, solo riding, collecting badges and virtual gear etc. You don't have to like it all, but there is enough variety to keep most people motivated while riding indoors.

Rouvy - basically video based virtual routes. If you prefer a more real world experience vs Zwift's "gaming" world then Rouvy is well worth checking out. It's pretty cool actually riding iconic big mountain climbs, especially with a decent smart trainer.

Wahoo SYSTM - This provides my structured training plans and I find the video workouts both motivating and effective. This is my main focus when training for specific events during the season.

Training Peaks - I have a premium account and previously used this for my structured training plans and analysing performance metrics, but now I'm using SYSTM for training plans I will probably drop my premium account when it expires.

Strava - I have a premium Strava account too. I find it useful for logging all my workouts in a single place (outdoor rides, Zwift, Rouvy, SYSTM workouts all sync to both Strava and Training Peaks), although I find the performance metrics very limited. It's also good for keeping track of what your friends and rivals are getting up to. I can't imagine not using Strava to be honest, but the value of a premium account is certainly debatable.