SharePoint Online Document Library READ permissions

I am confused with the security permission within SharePoint online document Library.

My Visitors have been assigned READ only access to the library.

Scenario ONE: Document Library set up as Modern Experience.

when the user clicks on the document (which is intended to be a template), it launches in the browser and can save changes back to the document. This is undesirable

The must right click and download to make changes locally.

Scenario TWO: Document Library set up as Classic Experience.

when the user clicks on the document (which is intended to be a template), it launches in the browser and cannot save changes back to the document. It prompts the user to select a location on the local machine. This is desirable

Why the difference? It is frustrating. I expect that if I set permissions as Read only for the library, then this should be observed in all instances.

To make matters worse, If I create a PowerApps Gallery interface for the library and use Download(ThisItem.'Link to item') it does not download. There is no difference between Launch and Download in power apps.

I have solved my issues by not using a Document Library at all. I created a Modern List, where my documents are attachments. Then I created a PowerApps customization of the list form, and when the user clicks on the attachment it downloads. But this does not solve the fundamental problem with Modern Document Libraries READ only permissions.

If any one has any ideas most welcome. By the way, even setting document properties and security to Read only does not make a difference to the editing behaviour online.