Learn Portuguese (Portugal): O povo saiu à rua para celebrar. - The people took to the streets to celebrate.

O povo refers to a collective group of people united by culture, nation, or a common cause (similar to "the public" or "the masses"). As pessoas simply means multiple, distinct individuals. Because we are talking about a collective social action, o povo is the natural choice here.

Since "the people" implies many individuals, why is the verb saiu in the singular?

In Portuguese, grammatical agreement strictly follows the form of the noun. Because o povo is a grammatically singular word, its verb must also be in the third-person singular (saiu), regardless of how many actual individuals make up the group.

The English translation says "to the streets" (plural), so why is it à rua (singular) in Portuguese?

The phrase sair à rua (literally "to go out to the street") is a fixed idiom in Portuguese used when referring to taking to the streets to protest or celebrate publicly. Portuguese uses the singular rua to represent the public sphere in general, whereas English prefers the plural "streets".

Could we use the personal infinitive celebrarem here instead of the standard infinitive celebrar?