Its very much the standard Coggan stuff to calc CTL (fitness) and ATL (fatigue) except that I use TSB (form) = CTL - ATL whereas some other tools use TSB = CTL_yesterday - ATL_yesterday.
CTL_decay = exp(-1/42) (or whatever fitness days is set to)
ATL_decay = exp(-1/7) (or whatever fatigue days is set to)
CTL_today = CTL_yesterday * CTL_decay + training_load * (1 - CTL_decay)
ATL_today = ATL_yesterday * ATL_decay + training_load * (1 - ATL_decay)
Strava doesn’t track FTP over time which makes a mess of the fitness chart. So maybe its that? When I first built the fitness stuff I did compare to Strava and got identical results. Since then they have added in “relative effort” and I don’t know how that works.