Long story short. I'm setting up a Sharepoint site and discovered Sharepoint Lists which I thought I could make use of. So I created a list with numerous columns but the default form to view and edit the records wasn't easy to navigate or understand. It was at this point I discovered the Customise Forms under Power Apps. That was 6 days ago. Before then I'd never heard of Power Apps so sorry but you are talking to a total newbie.
Anyway, I managed to create something in Power Apps and was just testing it out when I suddenly get "This app uses features that come with a Power Apps premium plan" and telling me to start a trial. For the past 6 days it's worked fine but suddenly today it's started happening. I don't know what license I have as it's part of my companies corporate account but I presume it's standard. I've checked what connectors are used in Premium https://us.flow.microsoft.com/en-us/connectors/?filter=&category=premium and I'm not using any of them. Everything I'm using comes from the Sharepoint List and these are all either text or number values.
My first thought was how do you identify what premium features are being used and why would it let you add them if you don't have a license for them? But also why has this just started happening. I've not changed anything other than changing the order of datacards on the screen.
The error message is probably a false positive if you are customizing forms in SharePoint online and that is the only connector you are using. If that is the case I would open a case with Microsoft. Their help will be needed to fix the issue. But to answer your other question you can navigate to the connections pane in the editor and see what connections are being used in the app.
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