all of the above and none of the above. In my mind, 1920 is not old, but getting there. Many of the finishes used in mass production will degrade. Most have become gummy, dirty, etc. There is little that will clean them except stripping. And that can be done carefully. You need to figure out what the old finish is.
Shellac will quickly dissolve in alcohol. Lacquer will soften in lacquer thinner. Varnish is probably one of the old finishes that is stable, but even that will dissolve with something like Formula 409 after 80+years. There were many proprietary finishes used that have no easy definition.
If you want to make the furniture usable, strip it, but do it gently. DO NOT send it out to a stripper. You won't recognize what comes back!!!!