For server configs, a LibreOffice Writer file per machine (except for RPs, I only have one for those), written as a didactic manual explaining how to install and configure everything (I work on bare metal still).
I started that way since diving into self-hosting was also a way to learn Linux, Sys-Admin and web-hosting. I don't do anything without updating the relevant chapters, or creating new ones.
Not gonna lie: it's tedious. But also a life-saver, and the rationals for my choices remain available years later, which is priceless in many ways.
Once upon a time I had neat network gear running, and I mostly YOLO-ed the doc for those, relying instead on the firmware/config backups. I had to put those devices away, but when I finally get to play with them again, I'm going to suffer re-learning and re-discovering everything.
Recently, I got to hack and old console, and just did a chronological log-file with actions taken and URLs to guides, instead of writing down everything myself. It got me thinking I might add a simple log-file to track my actions, on top of my usual guides.
Keepass (Windows) + Keepass2Android + Nextcloud has ben my winning combo for years. Recently switched to a Linux desktop and went with KeepassXC: still happy, except for Wayland not letting you AutoType (should be solvable though, but I lost interest at "reinstall X.org"...)
That must be a "Paris" in the Netherlands...