“high-intensity functional training”
Is not HIIT. HIIT is a single mode (run/walk...cycling etc.)
HIFT combines classic aerobic/anaerobic HIIT with resistance training like dead lifts, curls, bench press etc.
One after the other. Keeping your heart in the green/yellow/red zones.
Crossfit.
It’s the most intense exercise you can do.
Have heart issues, but fasting seems to help.