What you are seeing is the interplay between Signature Decay, Training Load (TL), and Breakthrough (BT) adjustments when decay meets your next activity that includes some maximal efforts under fatigue that may generate a BT.
If you zoom in on the chart the story is told through the matchsticks where blue is low strain (below TP) with high and peak strain (abovie TP) is represented by yellow and red bands.
All blue and rest days result in more decay but you can change the decay method if you prefer signature changes more closely align with changes in TL only. You might do that for extended base training periods (mostly low strain) after you’ve established your starting signature is in the right ballpark.
Xert is constantly updating your TP after every activity or lack of activity (decay).
TP isn’t FTP but serves a similar purpose. The numbers are close for many users but they are not measured in the same way. Traditional FTP tests are not required with Xert.
BTW Xert excels at training outdoors and analyzing those efforts whether you free ride or roughly follow a structured workout outdoors. That plus the Zwift integration options will provide everything you are looking for.
Lots of newbie tips here – Onboarding steps for Xert trial users and newbies - General - Xert Community Forum