Learn Finnish: Tämä taso sisältää sekä kahvia että teetä.

Sekä ... että is a fixed pair of words that works exactly like "both ... and" in English. You place sekä right before the first item (kahvia) and että before the second item (teetä).

Why are kahvia and teetä in the partitive case?

There are two reasons converging here. First, coffee and tea are uncountable mass nouns being talked about in an unspecified amount. Second, the verb sisältää (to include or contain) naturally takes a partitive object because containing something is viewed as an ongoing, continuous state.

Is sisältää the dictionary form or the conjugated "it includes" form?

In this sentence, it is the conjugated form ("it includes"), but it looks exactly identical to the dictionary form! The infinitive is sisältää. To conjugate it for a "he/she/it" subject like tämä taso, we take the stem sisältä- and double the last vowel, which brings us right back to sisältää.

What kind of "level" does taso refer to here?

Taso is a very versatile word. Depending on the context, it could mean a physical level like a shelf or floor, a difficulty level in a video game or language course, or a figurative standard of quality.