As others have said, Google Sheets for me. It's simple, multi-platform, cloud-based, etc. I have a tab for work performed with the columns you'd expect to find, and then a tab for upcoming work with things like symptoms, part numbers, links to walk-throughs, estimated cost/time, etc.
I also keep physical copies of the receipts for pretty much everything from work done at a shop (handy for warranty purposes) and parts purchased. The file itself is getting a bit big, so I might cull some of the stuff that is out of warranty and won't matter if I have. However, if I thought I'd be selling it at some point I probably would just keep them all for the next owner.
Good luck and welcome to the wonderful world of meticulous record keeping! Your loved ones won't understand...until that one time...:banana: Yeah, we're nuts.