Learn French: Le vote a été unanime, tous ont approuvé le projet. - The vote was unanimous, all of them approved the project.

In this sentence, tous is used as an independent pronoun meaning "everyone" or "all of them". When tous acts as a pronoun, you must pronounce the final 's' (to sound like "tooss"). If it were an adjective followed by a noun (like tous les votes), the 's' would be silent.

Why is it tous and not tout or toutes here?

Because it refers to "all of them" (multiple people), it has to be plural. It takes the masculine plural form tous either because it refers to a specific group of voters that includes at least one man, or because it is being used generally to mean "everyone," which defaults to the masculine form.

Why use the passé composé (a été) instead of the imparfait (était) for "was unanimous"?

While descriptions in the past often use the imparfait, here a été treats the vote as a single, completed event with a definitive result. The voting process concluded, and that conclusion was unanimous. If you were describing an ongoing background state, you would use était, but here the focus is on the finished action.

Does the adjective unanime change if it describes a feminine noun?

No, because unanime already ends in an -e in its base masculine form, it stays exactly the same for feminine singular nouns (for example, une décision unanime). You would only change it by adding an -s if the noun were plural.