I recently moved and I have a water filter that I'm trying to install under my sink. I'm used to flex lines, but this place has a piece of copper pipe running straight down from the cold water knob to the shutoff valve. There's a compression fitting on the shutoff valve, but I can't remove the pipe as it's screwed into the faucet above and I don't have a basin wrench. Additionally, I'm not sure if there's any soldering that goes on there so even if I buy one I don't know if that is another roadblock. Trying to figure out what I need to acquire to do this, and if I end up having to cut the tube or the like, what sort of connectors help me attach a flex line to the existing copper tubing.
Also is there a way to know the sizing of things from the diameter of the nuts (e.g. if I do get the copper pipe removed and can just slap a flex line in there, I need to know what size the threads are. It would be nice to not have to make a bunch of separate trips to the hardware store)?