I have a document library which has an infopath associated with it.
When I click on new button , it show popup with options for a brief before navigating to infopath form.
It seems you add a infopath form as a content type to the library. If you want to have a form for the library, you will need to go Library settings> Form settings> use a custom form created in PowerApps. You will need to use PowerApps for a custom form