The list form customized for SharePoint can't be displayed in the mobile app, it can only be displayed in SharePoint in a browser. There is also no way to directly export the form from the customized list to a Power App. You'll need to create a Canvas app and recreate the form. You may be able to cut and paste some of the controls from the custom form to the Canvas app, although that won't always work.
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Paul Papanek Stork, MVP Blog: https://www.dontpapanic.com/blog
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