Hi @4th8
Go to the Note entity in the maker portal:

In order to see it you will probably have to set the view in the top right to "All" instead of "Default"

When in the Note entity, select Relationships. Once again select the "All" view from the top right so you can see all the relationships. From here you can see which other Entities lookup to the Note entity.
If the entity you want to use Notes for isn't in the list, then you need to enable Notes for that entity. Go to the entity (in my example, I go to my Employee entity) and open it in the maker portal. Along the top select Settings:

In the settings click the box for Enable attachments (including notes and files) and then save your changes.
Now your entity has a lookup to the Notes entity.
If you add your entity to a Power App you will now see the Notes field in the list:

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