IT'S = IT IS, always. When you use IT'S - with an apostrophe, it means IT IS. ALWAYS, NO MATTER WHAT.
Examples (the first two are incorrect; the last three are correct):
The dog lost it's (it is) bone.
The site is notable for it's (it is) collection.
It's (it is) a story of two cities.
We think it's (it is) easy.
It's (it is) only a dream.
I do understand why people do this: that fuzzy rule about possession, i.e., use an apostrophe to show ownership.
Its = possessive pronoun, that means it was created specifically so that you wouldn't have to use an apostrophe to show ownership.
The dog lost its bone.
The site is notable for its extensive collection of links to resources.
It's not so difficult, is it?
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