In the mobile app you can add exercises per session.
But it cannot be save so can add it for next.
Its time consuming to add.
More over , there is no info what exercise is as many have many names.
My proposal is that garmin add a feature on the web where you can 'configure' strength exercises , like the intervals and should be able to
be one which is ready and a new manual on the mobile app. On the web there can simple pictures showing it.