Complex sentence: Although little is known about his early education, it is known his priestly studies were done in Siena but in his early twenties Albani returned to Rome to be a domestic prelate for Pope Clement XIV.
Split into simpler sentences: Although little is known about his early education, his priestly studies were in Siena. However, in his early twenties Albani returned to Rome to be a domestic prelate for Pope Clement XIV.