Learn Finnish: Tämä aula on hyvin kaunis. - This lobby is very beautiful.

Yes, hyvin is the adverb form of hyvä (good), so its primary meaning is "well". However, in Finnish, it is also extremely common to use hyvin as an intensifier meaning "very", similar to tosi or erittäin. In this sentence, hyvin kaunis means "very beautiful".

Why is the adjective kaunis in the basic nominative case instead of the partitive (kaunista)?

The case of the describing adjective (the predicative) depends on the subject. Aula (lobby) is a singular, countable noun. When describing a singular, countable object, the adjective stays in the nominative case. You would only use the partitive here if the subject were an uncountable mass noun (like coffee or water, e.g., vesi on kylmää).

How would this sentence change if we were talking about multiple lobbies?

You would need to make the subject, verb, and adjective plural. Tämä aula becomes nämä aulat (these lobbies), and on becomes ovat. Crucially, when describing a plural subject, the adjective must go into the plural partitive case. Kaunis becomes kauniita, making the sentence: Nämä aulat ovat hyvin kauniita.