Online, the answer is Arena. You can get started for free and if you never want to spend anything it’s possible to grind the resources you need to acquire new cards and compete in tournaments. You can get all the way to money prizes this way. Constructed formats are probably more economical than draft because you don’t have to constantly pay for packs.

In real life, every player has thousands of cards they’d happily dump on you if you just want to learn and have fun. You won’t be able to build a very competitive deck (or one up to date with current formats) but it’s fun to play over the table with friends.

The best investment for physical cards is probably the Commander format. This is a particularly social, multiplayer format, where all cards are legal (except for powerful stuff that’s been banned). You can build a deck over years and it’ll always be valid (if not good). This is also available on Arena, but most of the fun is getting round a table.

If you want to get competitive, there’s lots of content out there and many popular streamers to learn from. Magic Online is probably where the highest skilled players still play and test for tournaments but you’re unlikely to go infinite there.

If folks want to get into paper magic, the commander pre-constructed decks are a solid place to start. Some are almost competitive and require just a few tweaks to get into much better shape. They do a good job of packaging the thematic elements as well.