SAFARI TABS CLOSING BY ITSELF THE BILLIONTH TIME

How many tabs do you have open when this happens? Each tab needs an allocation of memory, and if your tab count is getting too big you might be exhausting the available resources. At that point it is likely Safari will either try to manage the memory usage by closing tabs you have not used recently, or it is possible some tabs might simply crash. If Safari is closing tabs to manage a resource shortage it is probably Working As Designed, and the only way to prevent this would be to upgrade your hardware for a device with more resources in the pot.

There are better ways to maintain a record of important or regularly viewed pages in Safari. Try setting bookmarks for the most important, or close tabs you don't need immediately and find them again in the History (or bookmarks).

Are you certain that you closed all your tabs - and have not, perhaps, opened a new Safari Window? Each Safari Window has its own self-contained set of open tabs.

When you open Safari, you may see the Shelf at the bottom of the screen - within which you will see each of your open Safari Windows; simply tap the thumbnail corresponding with the Safari Window (with associated tabs) that you wish to view.

If you closed a Safari Window - and all associated tabs - you can reopen a Recently Closed Window for a limited period of time…

Starting from a Home Screen, assuming that Safari is visible in the Dock, touch-and-hold (i.e., long touch) the Safari icon; an Action Menu will appear. 

From the menu, select Show All Windows; you will now see a screen that closely resembles the Task Switcher - but you will only see preview windows for your open Safari Windows. At top-right (or top-left for iPadOS16/17) you will see the Reopen Closed Window(s) button.

You should note that the Reopen Closed Window(s) button allows you to reopen a window for a limited period of time. If left too long, you may have lost the opportunity to recover the closed window - and with it the tabs and related tab history.

If you have accidentally closed all you your open tabs associated with a Safari Window, this User Tip may be helpful in recovering your tabs. While I have not [yet] updated the document to reflect changes in iPadOS15/16/17 (having been written for an earlier version of iPadOS, much of it is still relevant - as are the outlined strategies:

iPadOS - Safari - Lost Windows, Lost Tabs… - Apple Community