I have had a Zenit TTL for more than 20 years.
This camera is maximum unificated with Zenit B, BM, E and EM. All these are from the same series with different light metering. B and BM did not have a light meter at all, E and EM had built in light meter with sensor placed on pentaprism (not TTL).
The difference between B and BM, E and EM is automatic apperture that lets focusing with open aperture. The most advanced camera from this series is Zenit TTL, it has all best features from it's predecessors plus TTL metering. View finder is very dark (even darker than it's predecessors) that makes focusing in low light very difficult and shows only 67% of the frame.
Light metering is done with stopped down aperture using CDs cell that has very slow reaction and very long memory, it makes very difficult to take pictures in fast changing light conditions, it's match needle type. The camera was produced in two facilities: KMZ the better one and BeOmo with lack in quality control. Total production of Zenit TTL was about 6 million units.
The better version of Zenit TTL was Zenit 12XP that was made for export and had better body finish. The camera has very simple construction, it's highly upgraded pre war Leica. The good point is monolithic body construction that makes this camera extremely durable. Because in end of 70' already colour photography started to be more popular Zenit should ensure maximum accuracy of shutter speeds, so they replaced few metal parts with plastic that does not need lubrication.
Shutter movement was really smoother but the drawback of such solution was highly decreased shutter MTBF. The standard lens was Hellios 44 58mm f/2.0 a copy of Carl Zeiss Biotar. Zenit B, BM, E, EM had the same standard lens but another option was very small Industar 50mm f/3.5. Hellios 44 is 6 element lens that had multicoated version for luxury variations (for export and etc). Zenit 122 is the same TTL with plastic covered body, bright viewfinder, more accurate metering using 3 LED's display and few more small improvements, but generously the same.
Most Zenit SLR's have M42 screw lens mount, but there was a version with Pentax K bayonet. The older versions like mine don't have hot shoe' only a PC sync. This simple mechanical camera with very basic features is good as starting point for amateur that one day will become professional. The quality of photos is highly dependable on photographer. With good lens in good hands you can make a miracle.