Maintenance logs... How do you keep track of services?

I keep my cars til they're ready for the scrap heap - anything "major" like spark plugs etc I'll write on the air cleaner box, oil changes are kept track of using the sticker on the windshield.

Even when selling a car I've never had anyone ask for maintenance history.

I get the point of keeping records, but some people hilariously over-do it, no point in writing when you bought wiper blades or put a plug in your 7 year old tire , or bought a touch up pen for the 2 inch scratch on the lf fender (yes I've seen that in someones log book)🤷
Word document on home computer with basic info for each entry: date, mileage, description of service or repair, other notes going forward if it was not completely resolved. First page of these documents is a full writeup of the vehicle when I first bought it. I also have a piece of paper on my desk with handwritten date & mileage of latest basic services for each vehicle, used as quick-reference when I start wondering if service is due (or coming up soon).
Most manufacturers have a service history webpage where you can add information based on the VIN. Some garages, dealerships and owners update this history, some do not... generally it's pretty spotty what you'll find there. I've used this service history tool on the Chevy owners and Toyota owners websites. I still keep receipts.
"My Subaru" is an app that is part of their Starlink. It automatically adds service done at a subaru dealer. If you or others do the work you just enter it in there. It is the same app that monitors all your systems. Pretty neat.
I keep a simple three column Excel document for the friends/family cars I maintain:

I simply write it in the owners manual with pen, just the mileage not the dates.
Just a spiral notebook.All cars get written down in same logbook by date and any maintenance item. Very useful for the "lifetime warranty" parts chain items you will forget when you bought them, that will probably fail before you get rid of the car. I currently have 17 years of info on a Fox Mustang, including detailed rebuild items, which gaskets,parts,stores,machinists I used. Really handy, because you won't remember everything.
I use a silver sharpie and write under the hood by the radiator, the date, type and miles for oil and trans changes. It's crude, but I maintain so many things, it's the easiest way for me.

I also write the drain plug wrench size and oil filter part numbers by the radiator.
I use a Spiral notebook, stored in my garage desk for all maintenance logs
I use trip meter #2 for tracking oil change intervals as a record that is kept with the vehicle.
I try to change the oil on the 5's or as close to it as possible. It makes it easy to know where in the cycle I'm at and if the engine uses any oil, I can track the rate.
It's interesting to read this thread, I like to get ideas and try them for myself. I don't mind if it's a repeat, as both technology and BITOG responders change over time.

I document by:
1. Physical receipts for parts and services into a file folder, often with handwritten notes
2. Tag all email receipts "receipt" and "auto" in GMail.
3. Every maintenance/repair into a LibreOffice spreadsheet tab, every parts purchase in the next tab. One spreadsheet per vehicle.
4. A LibreOffice word processor document with notes, procedures, links to info
5. A filesystem folder per vehicle with the above spreadsheet and notes file, TSB's, my pics, etc, shared via ownCloud so I can edit anywhere.
6. A Firefox Bookmark folder for that vehicle with 3 sections- howto's, parts sites (primarily OEM), Aftermarket parts
7. A file backup scheme that borders on paranoia.

I tried an App briefly for auto stuff, it failed big-time for me because access/update via laptop browser was promised, but was awful. When I sit down at the end of a job and my phone doesn't even recognize my obliterated fingerprints and I can barely touch the right tiny portion of the screen "keyboard", is no time to enter 400 words of notes into a touch screen. I need my laptop, where the keyboard is real and doesn't care how beat-up my fingers are as I type 75 WPM.

I tried an OPE cloud site (B&S) where I entered all my equipment serial numbers and maintenance notes. They lost all my data. I asked their support and they said sorry, they had a failure, you'll have to re-enter your data. So I re-entered it, into a LibreOffice document. That was years ago, but "never again" on the somebody-else's-cloud. I have ownCloud for personal sharing between my laptop, desktop, and phone. If it goes down for a while, all the files are still there on each device.