ffmpeg – obtain RTSP Describe info and screenshot

good morning, given an rtsp streaming that responds 200 to a request and has both PLAY and DESCRIBE options, how can I capture a screenshot in jpg without running into errors like 400 Bed Request, using a command line tool like ffmpeg?
there are other answers on the site but without knowing the nature of streaming how can this operation be best carried out?

I found very little documentation online

I tried with ffprobe and ffmpeg, in tcp and udp, but each camera has its own options and before using ffmpeg I should understand what the streaming settings are.