iOS/iPadOS 14.5 will allow airplay to your tv if your tv is new enough to support the newer version of AirPlay (AirPlay 2.0), though without the AW metrics overlay. If yoyr smart tv only supports the older AirPlay (1.0) then you still won’t be able to AirPlay the Fitness+ videos to the TV.Does that allow screen mirroring? I can’t work out if my tv has airplay or not, but I can mirror from the iPad to fire stick using a third party app on the stick
Do note that for it to work you have to start airplay from within the Fitness app (button is on the bottom right when the fluorescent yellow play button appears on your iphone or ipad screen as soon as your watch is detected and right before you start the workout), not from control center.