Learn French: En été, il fait très chaud.

In French, seasons that start with a vowel or silent h take the preposition en. So we say en été (in summer), en automne (in autumn), and en hiver (in winter). Because spring starts with a consonant, it uses au printemps.

Why does the sentence use il fait instead of il est?

When describing the weather, French uses the verb faire (to do or make) rather than être (to be). So il fait très chaud translates literally as "it makes very hot", which is the standard French way to say that the weather is hot.

Does the word chaud ever change its spelling here?

Not in this sentence. When using the impersonal weather expression il fait, the adjective that follows (like chaud or froid) always stays in its default masculine singular form. It does not change to match the season.