Learn Polish: Gdyby portal działał, widzielibyśmy te same gwiazdy. - If the portal worked, we would see the exact same stars.

It describes gwiazdy (stars), which is a plural noun. Because stars are not a group involving men, we use the non-masculine-personal plural form te same. If we were talking about a group of men, it would be ci sami. In this sentence, it is the direct object of the verb, so it takes the accusative case, which looks identical to the nominative here.

Does te same naturally mean "the exact same"? I don't see a separate word for "exact" here.

Yes! The phrase ten sam (and its forms like te same) inherently means "the exact same" or "the very same" identity. You do not need to add an extra adjective to translate "exact" into Polish in this context.

Why is the verb działał in the past tense if we are talking about a hypothetical present or future?

That is how Polish builds conditionals (the "if... would..." structures). The word gdyby (if) requires the verb that follows it to be in its past tense form. Since portal is a masculine noun, we use the masculine past tense form działał.

How is the word widzielibyśmy (we would see) put together?