If you want to know how to SEE placements in PMax, check out this walk-through article (it's not complicated!): How to Find the Placement Report for Google Performance Max Campaigns.
If you want to exclude placement from just single PMax campaigns, you can now use this handy-dandy exclusion form from Google (also works for Keyword exclusions from individual campaigns!):
Step by Step Guide for Excluding Placements in Google Performance Max Campaigns:
Step 1 :
Select a PMax campaign > Content > Placement Exclusions tab in the left hand nav, as seen here (note, adding in exclusions to a single campaign, will add them in at the ACCOUNT level):
Click the Blue edit icon. Select the "Enter" option at the top. Add desired placement. Save.
Rinse and repeat for as many placements or YouTube channels you want to exclude!
And don't forget to exclude Mobile Apps from your PMax campaigns as well.