Learn German: Der Bericht bezieht sich auf das Jahr 2020.

In this sentence, we're using the fixed phrase sich beziehen auf, which means "to refer to" or "to relate to". If you drop the reflexive pronoun sich, the verb beziehen changes meaning entirely—usually to "to obtain" (like a salary or goods) or "to cover" (like making a bed).

Why does auf take the accusative case (das Jahr) instead of the dative? Isn't it a two-way preposition?

While auf is a two-way preposition when talking about physical location, that rule doesn't apply here. When auf is a fixed preposition tied to an abstract verb phrase (like sich beziehen auf), it almost always forces the accusative case, regardless of motion or location.

Is it strictly necessary to include das Jahr, or could I just say ... bezieht sich auf 2020?

You can absolutely just say auf 2020. However, adding das Jahr before the specific number is very common in formal written German, such as news articles or business reports, because it sounds more precise and professional.

How would I form a question to ask "What does the report refer to?" using this structure?