Items as created via .items file do not show up in paper UI

Setup, Configuration and Use Items & Sitemaps
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First of all, openHAB 3.x does not have PaperUI, this was the UI used in openHAB 2.x
In openHAB 3.x it is called MainUI.

What You descibe is normal, as your .items file(s) do(es) not contain any semantic tagging, therefore you will not see those Items in the model, unless you use the little tic box at the bottom.
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Then you will see all items.