Technically, Fitness and Fatigue aren’t simple 6-week and 1-week averages, but are exponentially weighted averages, so more-recent events have a bigger impact and events longer than 6 (or 1) weeks ago still have an impact, just small.
At least, I assume Strava’s Fitness is based off of CTL, which in turn is based off of Bannister’s TRIMP – since that’s what everyone does. I don’t know if TrainerRoad is using simple averages or exponential weighting, but for their purposes, I don’t think the two are all that different.
Using the Calendar TSS graph, you can visually estimate Form. The grey line on the graph is 6-week average TSS, which is roughly Fitness/CTL. The green bars are one-week TSS, which is roughly Fatigue/ATL. Form/TSB is the difference between the two. So, green bar above the grey line is negative Form; green bar below the grey line is positive Form.
You can work it out numerically, too, but note that Fatigue and Form change substantially in just a few days, so trying to pin an exact number on those using data that has a granularity of a week isn’t necessarily productive.