This app was mentioned in 3 comments, with an average of 4.67 upvotes
I use this app:
Android, free to install, no special permissions, no ads.
Are you on iphone or android? For android I use tabata HIIT timer. It's free, and doesn't take up much space on the phone. Most tabata apps assume that you'll be doing your own exercises, they just do the timed reps. That's fine with me, because I'll jump rope, do pushups and jumps. Do a few videos on YT and see what moves you like.
Tabata HIIT timer on android is simple and works well.
https://play.google.com/store/apps/details?id=com.xquadro.tabataTimer
So simple it hasn't been updated since 2018. No ads.