It’s up to each musician how to spend the practicing time, what and how to rehearse. Have time sight-read some new Mozart opus and transpose at the same time - extremely good for you.It just kind of happens. I don’t think anyone sets out to systematically learn how to do it. It starts off with the easy intervals first: seconds, fourths, fifths, then after a while you find you can even do it with harder ones (thirds, sixths, tritone)
Just no need to say that only (!) if you play that opus in all keys only then you learned it.