Okay - on the basis of what we can see in your screenshot, you have a slide-over Safari view (instance) associated with your Mail view (instance). When you open a link within an email, the link opens within the slide-over view of Safari that appears on-screen.
Without delving too much into the many UI concepts within iPadOS, suffice to say that you can have multiple “spaces” in which various multi-tasking Apps can be associated in either slide-over or split-view. In iOS 12.x, you had a much more limited version of what is now available in iPadOS 13.x.
If I understand correctly, you simply wish to dissolve this association between Mail and the slide-over Safari view. There are many ways to do this, each with slightly differing outcomes, but for this example I’ll outline one...
Using your example screenshot as the starting point, touch-and-hold the grey handle at the top edge of the slide-over Safari window; pull downwards to change the slide-over view to split-view (shared with Mail).
You will now have 50/50 split view on your screen of Mail and Safari - with a central divider upon which is a another grey handle. Drag this handle over the Safari view, maximising the Mail window to full screen. That’s it, done, the association between Mail and the slide-over Safari view is broken - but the Safari “instance” is still running in its own (now separate) space.
You may find this guide will provide some additional helpful reading - as it explains a lot (but not all) of the wrinkles of Safari and iPadOS multi-tasking and windowing:
https://discussions.apple.com/docs/DOC-250002077
The linked article was written with other Safari-related features in mind - however, there is significant crossover in context of Safari views, windows, instances and spaces.
I hope this finally resolves you problem - and provides some additional insight into iPadOS and some of its extensive advanced features.
