Reachmahjong.com

Today I've been investigating Where Winds Meet. It sounds like a mahjong game but it's actually a free-to-play open-world action RPG with a Chinese fantasy Wuxia setting (like Crouching Tiger, Hidden Dragon) so there's lots of fighting, heroic leaps and running up tall trees, but also lots of dungeons, loot and a bewildering array of items and menus.

However it does have a playable mahjong minigame. This can initially be found within the game world at the inn in Blissful Retreat in west Qinghe where you meet Aunt Han. (also a good mahjong name!) You can also play on the ostentatious Mirage Boat during the overlong The Gilded Chase side-quest.
You pay five units of Commerce Coin to play one hand against three opponents.

The minigame uses a variant of Sichuan Bloody Rules.

Only the standard suit tiles are used. Winds, dragons and flowers are not included.
After drawing their starting hand each player must choose one suit to exclude. These tiles appear at the right end of your hand and should be discarded first.
You hold thirteen tiles to make a complete hand of four sets and one pair (or exceptionally seven pairs).
The sets can be sequences, triplets or quads. You can make Pong or Kong calls but you cannot call Chow to make a sequence.

A 20-second timer is applied on each move, even in single-player mode.

The game recognises 21 combinations but you can go out with a Standard Hand (no combos).
A basic win is worth 10 pts but each quad made adds to your score.

When you've finished building your hand you can declare a win with "Hu". This locks your hand but the other players continue.
Combinations apply doubling multipliers to the hand value and can be stacked. There are no limits.
The discarder pays for a direct hit and all three opponents pay for a self-draw win.

A hand continues until three players have declared wins or the supply of tiles is exhausted.
Afterwards a penalty is applied for any player without a ready hand or holding their excluded suit.

In the example below I think I got 20 pts off each player for making a quad, then scored 10x32 pts for the hand win.
(The "Root" combo is for having four identical tiles.)
There are a couple of player aids. Selecting a tile in your hand will highlight all matches on the table.
The game also indicates which tile/s to discard for a ready hand and shows your wait/s and how many of each are available.

You usually play against three human opponents but, if you drink from the strange wine outside the inn first, you and your opponents will adopt canine form (Shiba Inu with waggly tails).
Other minigames include Xiangqi (Chinese chess) endgames and wrestling in padded tigerskin suits! I've spotted Go boards but these are not playable.

After completing Chapter 1 the mahjong playspot at the inn becomes unavailable but you can access it again via a somewhat unconventional route which starts by interacting with the sword beside the tree east of Blissful Retreat's wine tower.

Chapter 2 can commence when your character rank is at least Level 32. (You'll need to complete breakthrough battles to achieve Solo Mode Level 4 to raise the level-cap to 40.) This allows you to travel to the Kaifeng region in the bottom half of the map where you can play mahjong with the ladies in the big garish tower at the riverside fairground in south-east Kaifeng City. You can't miss it, especially at night!
Kaifeng also has a minigame using Madiao cards. These traditional Chinese numbered playing-cards ultimately contributed to the development of mahjong - the deck has four suits including coins (dots), strings of coins (resembling bamboo) and myriads (ten thousands 萬). The minigame uses a modified Madiao deck with cards numbered 1 to 10 in all four suits and yellow-topped wildcards added which can take any value.
The game played in Where Winds Meet is similar to the card game Cheat/Bullshit. A player nominates a card rank (1-10) and discards a number of cards - supposedly all of that rank but they could be bluffing. Each player in turn discards some cards (which are again supposed to all be the specified rank) or they can pass. Immediately after each discard another player can issue a challenge - if a player is correctly challenged (some/all of their latest discards are not the correct rank) or a player issues an incorrect challenge, they take the whole discard pile into their hand.
Over further rounds each player tries to discard all their cards but there is an additional factor - passing a round or failing a challenge causes the player to take a drink. Each player has an initial 25% chance of passing out instantly from bad grog and this increases by 25% with each drink penalty. You win the game either by clearing your deck first or by all three opponents passing out.

Evidently this game is very popular in Kaifeng - there's half a dozen playspots within the city walls and another dozen in the rural areas beyond. Your first win at each location gives a bundle of rewards including a useful chunk of XP.