What is the difference between
The company opened {it’s / its} doors in May 2015, and {has since / since then} finalized all the contracts.
And which is better?
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Sign up to join this communityYou do not need the apostrophe here in the word "its". In your case it is a possessive pronoun derived from personal pronoun "it". So you must use its and not it's. It's is used when it represents a contraction from it is.
About the second part. Since then finalized is not very good. Has since finalized is much better.
So in my opinion your sentence will be like that:
The company opened its doors in May 2015 and has since finalized all the contracts.
(Present Perfect indicates a terminated action.)
I removed the comma before and. I think it's unnecessary.