I read somewhere living together as a couple is called "sambo", so is there a verb "sambor" = to live together?
It exists as in it's in the dictionary, but it's not used. You'd say "vi ska bo tillsammans" or something like that.
Does tillsammans literally translate to tosame(ly)?
Samman means basically gathered, so it has basically the exact same background as english TO+GETHER.
Like "zusammen" in German
De bor ihop is ok? What is the difference.
Is this irregular cause it doesn't have an a at the end although it's plural?
Tillsammans is an adverb, describing the verb not the subject of the sentence. So no endings for plural.