1. If its a bottom end bike (steel rims, cottered cranks, etc), I just donate to my favorite charity. I don't buy them, but sometimes I find them free (in the trash). They are not worth putting a wrench on because no one around here will pay anything for them. If there is anything decent on the bike, I will remove the parts for possible reuse later. If it is vintage (1970s) and in decent shape (and not a Huffy, Roadmaster, Free Spirit and similar, I will refurb it. It doesn't make sense financially, but I kind of have a soft spot for 1970s boomer bikes (thats what I grew up with).

2. On a decent bike (alloy rims, cromoly frame, three piece crank), I evaluate overall condition. Some are pretty good, so I might just replace a cable or two. Most (but not all) need tires and tubes. So while the wheels are off, I check the condition of the wheel bearings and almost always replace them. I check chain, clean it if it is good and lube it properly. BB is evaluated and possibly rebuilt. Ditto headset. Almost all need a new seat. And I put on new bar tape.

3. On keepers, I evaluate condition like number 2, evaluate what needs service and then just do that service, fully expecting to do more over time as routine maintenance. Example is the Colnago I bought last week. I replaced the out of place (wrong for bike) bars and stem, I installed new bar tape, I replaced one brake cable (it was dragging), I installed new tires and tubes and I cleaned and lubed the chain.