This app was mentioned in 7 comments, with an average of 1.43 upvotes
I use Let's Mahjong....it's the legit kind with Cantonese speaking sounds. You play with real people around the world. the illustration's are supposed to be 70's HK style too which is cool. There is also an option where you can play in real time against your friends.
https://play.google.com/store/apps/details?id=com.gdapp.mjlafree
paid version is $2.99 here
The game is called 麻雀 (like the bird).
Many games/apps available. Try this one for instance: https://play.google.com/store/apps/details?id=com.gdapp.mjlafree
Unfortunately I have no answer but I recommend just to start play. Afaik Let's Mahjong is one of the more popular HK mahjong apps
https://apps.apple.com/us/app/lets-mahjong/id933694165
https://play.google.com/store/apps/details?id=com.gdapp.mjlafree&hl=en&gl=US
It has some rules and terms explanations but no step by step tutorial. I play riichi and honestly only saw proper tutorial in one single game (Kemono) but even that is just scratching the surface because there are so many things going on in mahjong. That's why I recommend just to play, watch videos on Youtube etc.
dk if it will notify the rest of the commenters anot but here goes
https://play.google.com/store/apps/details?id=com.gdapp.mjlafree