Learn Spanish (Latin America): Llegamos al mismo tiempo.

This sentence is a great reminder that al still has its regular job: combining the preposition a (to/at) and the article el (the). Here it is followed by the noun phrase mismo tiempo (same time). You only treat it as an infinitive temporal clause (meaning 'upon doing something') when it is directly followed by a verb in the infinitive.

How do we know llegamos means 'we arrived' in the past tense and not 'we arrive' in the present?

For regular -ar verbs, the nosotros form is exactly the same in both the present tense and the preterite (simple past) tense. You have to rely on the surrounding conversation or the broader context to know which timeframe is meant.

Why is mismo placed before the noun tiempo instead of after it?

Descriptive adjectives usually go after the noun in Spanish, but mismo is an exception. When it translates to 'same', it almost always comes right before the noun it modifies.

Why do we use al instead of en el to say 'at the' here?

Prepositions rarely match up perfectly between languages. While English says 'at the same time', Spanish uses the preposition a for this specific phrase. The phrase al mismo tiempo is a fixed expression that you can just memorize as a single chunk.