What is the easiest way to keep maintenance records organized for more than one vehicle?

I help look after a few vehicles for family, and one thing I've realized is that remembering what was done on which car gets confusing faster than I expected.

Oil changes are easy enough, but once you start adding brake work, filters, fluids, batteries, and other maintenance, I end up digging through old notes or receipts just to check if something has already been done.

I'm curious what everyone here actually uses.

Do you stick with a notebook, a spreadsheet, reminders on your phone, or something else?

I'm trying to find a system that's simple enough to keep using consistently instead of letting everything end up scattered across different places.