The Russian air force seems to have been effectively smaller than expected in terms of their aircrews. Looking at the KIA/POW pilots in downed RU planes (100-ish?), initially it was surprising how many of them were very senior (like half of them being at e.g. lt cols/wing commander level) and afterwards it was surprising how junior they were (like right out of flight school).
A hypothesis was that this indicates that before the war only a limited number of aircrews got sufficient flight time (which, obviously, is quite expensive; especially if many of your planes - just as tanks - aren't properly maintained to save costs or due to corruption) and the same people were used in all the operations in the last decade; and once these air crews "ran out" there is a bottleneck in capable replacements.
But S-300 definitely was deployed not only within the borders of Russia but also by Russia in Ukraine, I recall seeing evidence of that - it's a defensive system that is also used to defend the deployed Russian ground forces from air attacks.
A hypothesis was that this indicates that before the war only a limited number of aircrews got sufficient flight time (which, obviously, is quite expensive; especially if many of your planes - just as tanks - aren't properly maintained to save costs or due to corruption) and the same people were used in all the operations in the last decade; and once these air crews "ran out" there is a bottleneck in capable replacements.
But S-300 definitely was deployed not only within the borders of Russia but also by Russia in Ukraine, I recall seeing evidence of that - it's a defensive system that is also used to defend the deployed Russian ground forces from air attacks.