Learn Hardcore Hausa: Na gano cewa Musa yana gida, sai na tura masa saƙo ta waya. - I found out that Musa is at home, so I sent him a message by phone.

Na gano = I found out / I discovered (completed action).
sai na tura = then I sent (another completed action that happens next).
It’s normal to repeat the subject pronoun when you start a new verb/clause.

What exactly does gano mean here, and is it always “to find”?

gano literally relates to finding/discovering, but in sentences like this it often means to find out / to realize / to learn (information). So Na gano cewa... is a common way to say I found out that....

What is cewa doing in the sentence?

cewa is a complementizer meaning that, introducing a clause that reports information:

Na gano cewa Musa yana gida = I found out that Musa is at home.
It’s very similar to English that in reported statements.

Why is it Musa yana gida and not something like Musa na gida?