Learn Finnish: Elämä on arvaamatonta, mikä vaatii meiltä itseluottamusta. - Life is unpredictable, which requires self-confidence from us.

We use joka when referring back to a specific noun, but mikä is required when referring to an entire preceding clause. Here, mikä refers to the whole fact that life is unpredictable, rather than a single word. This fits into the broader rule you are learning: mikä steps in for joka when referring to whole sentences, pronouns like kaikki, or superlative phrases.

Why is the adjective arvaamatonta in the partitive case?

In Finnish, when you use the verb olla to describe an abstract, uncountable concept like elämä (life), the describing adjective takes the partitive case. If we were describing a concrete, countable noun, the adjective would be in the basic nominative case.

Why does meiltä end in the ablative case (-ltä)?

The verb vaatia (to require or demand) requires the person from whom something is demanded to be in the ablative case (-lta/-ltä). You can think of it as requiring something "from off of" us.

Why is the object itseluottamusta in the partitive case here?

It is the object of the verb vaatia. Because self-confidence is an abstract, uncountable thing, the object takes the partitive case to express requiring an undefined amount, or "some" self-confidence.