Learn Italian: Bisogna incastrare il pezzo, ovvero fissarlo bene. - One must slot the piece in, namely fix it well.

It can, but not here. In this context, ovvero is an explanatory marker meaning "namely," "that is to say," or "in other words." It reformulates the first action (incastrare il pezzo) by describing exactly what it achieves (fissarlo bene). While ovvero is sometimes used in formal Italian to mean "or" as an alternative, here it introduces a clarification.

What exactly does incastrare mean here?

Incastrare means "to wedge," "to slot in," or "to fit together." It is used when you physically push a part into its correct place so it stays stuck there, like a puzzle piece or a mechanical component. As a fun side note, in crime contexts, incastrare also means "to frame" someone!

Is fissare a false friend for the English word "fix"?

Yes! While fissare translates to "to fix," it strictly means "to fasten," "to secure," or "to stare at." If you want to say "to repair a broken piece," you must use riparare or aggiustare. In this sentence, fissarlo bene means to secure the piece tightly.

Why is there no preposition between Bisogna and the infinitive incastrare?

The impersonal verb bisogna (meaning "one must" or "it is necessary") is always followed directly by an infinitive, with no prepositions in between. Be careful not to confuse it with the personal expression avere bisogno di (to have need of), which does require the preposition di.