Based on your description, I suggest you use highlighted content web part to automatically populate documents from document libraries in the different site collections.
Note: All users want to use highlighted content web part should have permissions for documents, document libraries and site collections.
1.In a page, insert highlighted content web part.
2.In the Source, select sites you want.
3.Choose desired content type and document type.
4.Set layout.
5.Result.
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