Learn Portuguese (Portugal): A sopa tem muito sal.

The word muito here means "a lot of" and it describes the amount of salt (sal), not the soup (sopa). Because sal is a masculine word, the quantifier must match it and be the masculine singular muito.

How do I know that sal is masculine so I can match muito to it?

Nouns ending in -l don't have a strict gender rule, so you just have to memorize that salt is masculine: o sal. This is why we say muito sal (a lot of salt) or pouco sal (little salt).

Where is the word for "of" in this sentence? (As in "a lot of salt")

Portuguese doesn't use an extra word for "of" after quantifiers like muito (a lot / much) or pouco (a little / few). You just put the quantifier directly before the noun: muito sal.

Why use tem (has) instead of saying the soup is salty?

While you can use adjectives to say food is salty, using the verb ter (to have) to say a dish "has" a certain ingredient is an incredibly common and natural way to describe food in Portuguese.