Poorly! I do it poorly! :D
Characters are in GURPS Character Sheet, an (awesome!) open source character management program. I run our games through Foundry VTT, which handles all the combat tracking, HP/FP management, maps when I have them, etc. It also imports GCS files (and GCA, if that's your flavor of character editor). Most of my other notes (events, generated people names for unexpected NPCs, etc) go in Google Keep notes. Notes that I have to take on the fly go into my notebook, where they go forgotten until the next session, more often than I'd like...