삼사합니다 and 고맙습니다 are the two main ways. The first one is from Chinese and the second is Korean only. But they can also be conjugated to less formal forms, so you may also see 감사해요 or 고마워 for instance. These are still the same basic words though.

I think there are? From my notes, i have 고마워 "thanks!", 고맙습니다 a formal thank you, and 강사합니다 a more normal everyday thank you.... I could be wrong though?

Yes. Polite, but less formal way.

Yes, just less formal

That's not quite how you spell thank you. 고마워요 한영해요

Its supposed to be 환영합니다, you mispelled it

You can pronounced it like you say "m". So in 감사합니다 you should pronounced: gamsahamnida instead of gamsahapnida

Like a b.

Like a b.

As explained in the notes here: https://www.duolingo.com/skill/ko/Alphabet-2/tips-and-notes

Some final consonants change their pronunciation depending on the starting consonant of the next syllable

In this case, when a final consonant ㅂ is followed by a ㄴ of the next syllable, ㅂ is pronounced as m

입니다 (im-ni-da) as opposed to ib-ni-da