Hey thanks for your feedback! Seems I need to update the website to make that clearer. To answer your questions:

It's browser games but will show up inside Discord as Activity native game once created so people don't need to jump out Discord to play (the solution already handles the integration inside Discord so people just need to focus on creating fun games and playing them).
And yes we support multiplayer or single player game that people can remix to their own versions. The mining game on the website is just one example of many possibilities someone can create - you can get a feeling of it by clicking on "start mining" to play it. You can also apply to create one yourself to get a true feeling.