Learn Finnish: Ravintola oli täynnä ihmisiä. - The restaurant was full of people.

When describing a state of being filled with something, Finnish uses the essive case form täynnä. It functions less like a normal adjective modifying a noun, and more like an adverb describing a state, which ties into the phrases of state you're learning in this lesson.

Why is ihmisiä in the partitive case here?

The word täynnä requires the word that follows it (the thing it is full of) to be in the partitive case. You can think of it acting like a measure word or quantifier, similar to paljon (a lot). Because people are countable, we use the partitive plural ihmisiä.

Is it also correct to say Ravintola oli ihmisiä täynnä?

Yes, completely! In fact, placing the partitive word before täynnä is very common and sounds highly idiomatic in Finnish. Both word orders mean the exact same thing.

Does this sentence contain an absolute phrase (irtonainen tilanilmaus) like kädet taskussa?

Not strictly speaking, because an absolute phrase typically combines a nominative noun (like kädet) with a local case or adverb (like taskussa) to show the state of a subject. However, täynnä is an adverb of state doing a very similar job in the sentence: it paints a picture of a static condition rather than an action.