[dnanian]

1. On a regular copy (10.15 and before on APFS), they're just normal files. I explain why here - http://www.shirt-pocket.com/blog/ind...aternal_twins/ However, on Big Sur, an erase-then-copy backup uses replication which will preserve the clones. Subsequent Smart Updates might or might not increase the cloned file sizes, depending on what files were affected, and how.
2. It'll be the current state of the files unless a source snapshot is selected.
3. We include and maintain them automatically. There's no need, in Catalina or earlier, to install.
4. No, but see Big Sur comment above.