Best to pair HR with Xert and only save the Xert activity when workout is finished and have Xert sync to Strava.
SMART intervals scale to your fitness signature and include a number of different options.
In the case of dynamic intervals either power or duration fluctuates during the interval depending on how well you are hitting and holding target watts (compliance). HR does not factor in and would be unreliable since HR drift (decoupling) occurs during most workouts as core temp rises.
Lots of newbie tips posted here – Beginner questions
Academy series videos are the quickest way to get up to speed – https://tinyurl.com/yfrucamh
FAQs here – Support Home – Xert
Welcome aboard. 