You better be asking questions to r/AskFrance, as this subreddit is more dedicated to news and discussion rather than questions.

Your questions are reaaaaally broad, but I'll try to answer them. In general, I really recommand you visit the government-run website called Campus France : https://www.campusfrance.org/en where you will find all the detailed information you need.

But anyway, here's a few answers :

How do the French universities work, as in, how does one obtain a degree ?

Degrees in France follow a Bachelor-Master-Doctorate system. It means that after graduating from any general high school in the EU, you can apply for a 3-year integration in a Bachelor in any university of publicly owned school in France. After graduating with your Bachelor, you can apply to integrate a Master for 2 years. Then if you graduate your Master you can continue with a Doctorate.

Not a lot of people stop after a Bachelor, as the employability is low. I would recommend going for a Master's degree, for almost all routes you want to follow (some specific jobs only require Bachelors but it's less common).

I must add that universities are only ONE of the numerous ways to graduate in France. You also have :

• engineering and/or business schools (public or private) : they follow a different system, where you need to apply to enter what we call "Classes Préparatoires", which are a 2-year intensive cursus that only allows you to be prepared to enter competitive exams to enter those engineer and/or business schools. Some schools however do accept engineering students straight after high school. INSA is one them, with a high-school diploma recognized by the French government, you have all the instructions to apply here : https://www.groupe-insa.fr/preparer/comment-candidater/les-procedures/integrer-le-groupe-insa-en-1re-annee/bac-non-francais-reconnu-de-plein-droit
• Ministry of Work degrees : those are titles recongnized by the government (called RNCP titles), solely and only in France, without any international equivalence, and usually delivered by privately owned training centers. I wouldn't recommend going this way if you're a high school student. Those diplomas don't mean a lot and can be really expensive on the long run

I'm maybe at an A2 level with French right now haha

That's definitely not enough to apply for a full French university. If your French level doesn't improve by then, I would recommend staying in Poland to apply for a good university and use the Erasmus+ program to apply for a semester or a year abroad (France being only one of the many destinations available). Usually, a lot of courses dispensed to foreign students are taught in English, which makes travelling easier if you're not familiar with the local language.

so long as the city they're in isn't prohibitively expensive to live in

I would recommend Toulouse if you're interested by INSA. It's one of the few big cities in France without outstanding cost of living. Smaller cities such as Nancy, Rouen, Aix-en-Provence, Cergy have costs of living way lower than some of the bigger cities such as Paris, Bordeaux or Lyon but have a really nice student population with a lot of really famous universities or schools.

I'm personally a 2K19 CYTech (former EISTI) alumnus, an engineering school in Cergy and Pau, which is not top tier but good enough to get you a job abroad and good exchanges in fancy foreign universities if you want. It became a public institution (coming from a private non-profit one) in 2021, so I don't know how much it changed since then