The meaning and the role of 'that'

The below is a very tricky sentence I met.

I analyzed it as much as I can.

But I can't figure it out.

Can you check it for me?

In other words, even though we often mess up, most of us are doing the best that we know how with the circumstances that surround us.

<From the book, "Don't sweat the small stuff" ch. 25>

What I want to ask is how should I understand 'that'?

Is it a conjunction, relative pronoun, or relative adverb?

In each case, what's its role and meaning?

Thanks in advance.


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