I remember doing combinatorics puzzles and number theory exercises in platforms like Brilliant when I had a bit of free time. All the while learning something. But the last time I tried Brilliant I did not enjoy it as much and I can't afford it not anyway.

Does anyone have any recommendations of websites (or webrings), apps or games about or that touch on:

• combinatorics puzzles
• modular arithmetic puzzles
• concepts or exercises of number theory