If the author is lurking here, would love to learn about the tech used to build it.
At the moment it's a single node.js server using socket.io for communication between clients and the server. The server holds the game state completely in memory, which is updated in response to game state changes sent by each client. The single-threadedness of node.js serialises all these changes to form a globally consistent game state, which is pushed wholesale to clients whenever it changes. Clients are stateless (almost), rendering only what the game state dictates. There's no database or user accounts, everyone just has a UUID assigned to them.
I've seen a large increase in traffic due to the current pandemic (142k users over the last 28 days, about 40x the norm), which has put a lot of stress on my little AWS micro (sorry about the outages). I've made a lot of performance fixes but ultimately I'll have to make this scale across multiple servers if growth continues. Current plan is to start games on whatever server they land on (load-balanced), and have a master task which knows what games are being played on what servers for the purpose of game discovery and room code collisions.
Oh and because I was a thoroughly mislead individual when I first wrote it, it's largely written in CoffeeScript... slowly migrating to Typescript :)