How does Teams filter for spam / junk?

You cannot really. Emails sent to Teams channels are attributed to a Microsoft-owned tenant (thus the @teams.ms address) and anti-malware/anti-spam filtering as well as any other processing happens there. As it's a different tenant and you have zero access to it, you cannot do the usual troubleshooting such as looking at message traces - at best they will show you the "internal" delivery from the MS tenant to yours. Best open a support case.