Spamassassin questions?

I have a some questions about how Spamassassin works on MiaB and Roundcube. Most (if not all) of my incoming email is filtered to folders by Roundcube filters, so very little email sits in my inbox. If this is the case, how does spamassassin train ham? Often the only email that is in my inbox is spam (before I move it to the spam folder). Roundcube seems to keep all the email in a sqlite db, so I’m not sure how I ever train the ham? Am I misunderstanding where all my folders are?

When does MiaB train spam/ham? Is this done just once when postfix sends it on? If it’s done periodically does it train ham in all folders, not just the inbox?

Most of the spam I get is all in Japanese or Chinese and so far spamassassin hasn’t caught a single piece of spam, so I’m starting to wonder if it’s working?