math

Is there an equivalent of a Duolingo for intermediate/advanced Math?

Fan of burritos, digging into unutilized capacity,

trying to solve some problems at https://www.louiza.xyz

Interested in ZK, FHE, PUFs, hardware, system design, ai safety, and smart contact security.

Ya same, particularly keen on an app with minimal friction that's conducive to building & maintaining momentum. 

Totally see how video's work for some folks but would prefer a more 'learn-by-doing' experience.

Best I've found so far is khan academy's calc and multivariable calc courses. I would love an app that just generated individual math problems at some selected difficulty and domain

Brilliant is quite good, but the pacing of the courses is slightly too fast if it’s your first time through the material. It’s great for filling idle time on topics that you’re already familiar with. Nothing beats pen and paper though imo. 
Also you have to pay.

Not Doullingo, but a useful interactive one:

Not Doullingo, but a useful interactive one:

https://brilliant.org/

There are Euclídea and Pythagorea apps for geometry. Played a lot several years ago. Not exactly what you asked, but math is very broad term...