How can multi-level page tables save memory?

Hello, everyone. I have been looking at the multi-level page table of the operating system recently, but I still have some doubts about the multi-level page table.

the first-level page table covers all the virtual address space, and some of his table items are not used, so there is no need to create a second-level page table for it. The second-level page table is only created when needed. Where are the used / unused first-level page tables?

if there is a secondary page table, is the top-level page table in memory, and the corresponding page tables (secondary page tables) are also in memory, while those unused page tables are on disk?

Aug.04,2021
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