Learn Portuguese (Portugal): Ele foi às urgências ontem à noite.

The verbs ir (to go) and ser (to be) share the exact same forms in the simple past tense. You have to use the context to tell them apart. Because it's followed by a destination—às urgências (to the ER/A&E)—it clearly means 'went' here rather than 'was'.

Why is urgências plural? Is he having multiple emergencies?

In Portugal, the Emergency Room (A&E) department at a hospital is usually referred to in the plural as as urgências (literally, 'the emergencies'). You will almost always see it used this way when talking about the hospital department.

What exactly is the word às made of?

às is a mandatory combination of the preposition a (to) and the feminine plural word for 'the', which is as. We use it here because urgências is a feminine plural noun. The accent mark points backwards to show you that these two little words merged together.

If the English translation says 'last night', why does the Portuguese sentence use ontem (yesterday)?

In English, we say 'last night', but Portuguese expresses this literally as 'yesterday at night': ontem (yesterday) + à (at the) + noite (night). Translating 'last night' word-for-word into Portuguese would sound very unnatural.