A lot of people still use paper (and fill it in after landing each flight), but there are quite a few digital options on the market now. I use one called LogTen, which stores everything in a SQLite file behind the scenes which is what I used to make this.
You go back to every airline you ever worked for, beg for your flight logs, hope they are willing to provide them, and then over the course of weeks re-enter them into your paper logbook or software. Then you hope that the airline will also be willing to sign-off your logbook.
You are only required to log time required for 61.51.a.1 or .a.2, but are not required to log “all [your] flying hours” by the FAA. (Your airline might require it and it’s a good idea to log all your flights, but it’s not a law.)
