I frequently use lock-in amplifiers and I'd be happy to help.
However, I am not sure the questions really make sense; they sort-of imply that there is a sensitive alternative to using a lock-in amplifier whereas in most of the situations they are used it is the only (sensible) type of instrument you could use for that type of measurement, it is rare for there to be an alternative to using a lock-in.
To see what I mean just imagine asking the same questions about say voltmeters.
One can of course discuss pros/cons of different models (or even say analog lock-ins vs. DSP based ones) but that doesn't seem to be what you are asking
I learned the basics with NE565 PLL and 567 tone decoder. I was able to FM modulate telephone DTMF signals onto inaudible ~25khz carriers and send them all over a power plant via the PA system without affecting its use for voice communication..
There are much newer and more complex IC's available now.
Good luck with your investigations. PLL's are a LOT of fun.
We normally do not allow survey/interview-type threads. But as long as this thread is confined to the technical discussion of lock-in amplifiers (and no personal information is requested), it may be okay. The Mentors will monitor the thread going forward. Thank you.