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!):
>> Performance Max Campaign Modification Request Form <<
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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):
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Step 2:
Click the Blue edit icon. Select the "Enter" option at the top. Add desired placement. Save.
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Step 3:
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.