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.