It looks like you're here because you have an Argentine girlfriend. If you intend to stay long term, sign long term rental contracts, obtain health insurance, and do most of the normal things, you need to obtain either temporary (and then permanent) residency or citizenship. Marrying your girlfriend may be the only way to do either. And eventually, you do end up having to pay taxes in the US and Argentina (or becoming a tax evader in Argentina and running the risks that entails).Talk to me like I'm super new to this. What do I need to do in Uruguay to keep my tourist visa?
Will I need to be legit to buy a car, sign a long term rent agreement, sign up for healthcare or get married in Argentina? Could not being legit interfere with my girlfriends visa application(to the US) or other visa/citizenship applications such as mine in the future?
What does the "legit" process look like and what are its limitations? Do I end up paying US and Argentine Taxes?
There is no easy way to answer your question and no one set of answers. But the first thing you need to do is work out whether you want to settle down here for good, or whether it's just for a year or two. (If it's just for a year or two, you may be able to get by leaving the country regularly, staying away for a few days (you don't actually have to DO anything in Uruguay or anywhere else) and hope they let you back in for another 90 days.)
Once your clear about whether this is permanent or temporary, start searching this website, because the answers (or at least a set of great clues) for either scenario are all here. Similar questions get asked all the time.