They mostly gather postions of AD and time response for the interception by listening to communications which must be not even encrypted.
Even newest radar work in peace time mode. They use only one frequency (without the ECCM with frequency hoping and what not) at normal power output.
You can't really gather anything from it. If the radar works in L band you just choose one frequency in that band and the enemy won't learn anything he didn't know.
So no one realky knows what radar they use because L band on a 2020 radar is the same L band as on a 1970 radar. Processing power can't be known and that is the most important about a radar.
To improve the quality of capture information... ie to trick the Soviets into real emergency intercepts rather than routine escorts they used to use converted bombers which were generally previous generation bombers converted to carry electronic equipment... and added an R to its code... so for instance a B-17 used would be codenamed RB-17 and from a distance it is just a B-17 bomber.
It used to be enough just to approach the soviet border to gather information but then the Soviets got cagey, so they often had to fly directly at the border and then turn away at the last minute to remain in international airspace... eventually that stopped working to so the started cutting corners where Soviet airspace poked out into international airspace so they would be inside Soviet airspace for a few seconds or minutes. Eventually they just flew into Soviet airspace.
Quite a lot of western crews were killed doing this and it led to the Soviets being suspicious of all aircraft entering their airspace which resulted in airliners getting shot down... remember most airliners are also used by the US military for various roles too...
Would be interesting to tot up all the people killed to collect all that information.