The new Performance Max ad type can be very confusing. But from what I have been able to gather, it gives you full access to the entire range of inventory Google has, across search, display, discover, mobile, etc. But setting up the ads can be tricky.
Ad Text
Here is a table of the types of ad copy you need for these ads:

(you can find the source from Google here.)
Image format
Performance Max ads allow you to add a bunch of text, as above, and a bunch of different image types, and them Google mixes and matches them for you. Here are the specs for those files.

Video Format
You are also encouraged to do multiple videos as well. You only need to start with a 10-60 second YouTube video, but Google ads does suggest to you do videos in all 3 orientations.

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