I don't know how to measure its degree of informality, but it is about as informal as communication among Finns tends to be. Colloquial forms are all over the place and speaking "properly" can seem unnecessarily formal. You can even refer to people as "se" (it) instead of "hän" (he/she) and they won't even be fazed by it, so you don't need to worry about being too informal. It's far more common to say "joo" or "juu" than "kyllä".
Hmm, when it comes to using "se" for people the "hän"/"se" divide didn't actually exist before the 19th century and it was completely normal and neutral to use both "se" and "hän" for people and animals.
Regional differences exist to this day and in certain places it's more common to use "se" and in others "hän". I personally almost never use "hän" when speaking. If I do, it's either because I'm being super respectful or a little sarcastic. In written Finnish, if it's something more official, I do use "hän" though.
I didn't even know that about the history, that's pretty interesting!
There's also this joke going around that we Finns use "se" when we talk about other people and "hän" only when we talk about animals/pets or the speaker mentions their own kid :D Talking of the regions, I normally never say "hän" but when I was working with certain people, I somehow adopted the talking style of the older generation (I'm speak the Savo dialect) and I started using my dialect's version, "hää", a lot more and especially its other declinations.
I read in the tips and notes section that "kyllä kiitos" is used mostly when the word "haluaisitko" is in the question, and to otherwise use "joo". Do i say "joo kiitos" or just "joo" in everyday situations, such as talking to the cashier? I don't want to seem rude but also not weird lol