multi line find in files and replace?

sorry, new to google groups in general, i dont know how to handle links properly.

https://reddit.com/r/vim/comments/buvs49/quest_noob_friendly_mass_select_copy_search
shortened TL:DR
select lines of text, copy to register, search for string in files, replace.
a look of what i want to do is shown in https://github.com/brooth/far.vim#multiline-replacement

from some digging around
https://vim.fandom.com/wiki/Search_and_replace
says we can use registers while using subs, but i somehow cant use <c-r> (and <c-v> in behave mswin) as both search and replace parameters. but, with \+@ i can freely use any reg as the replace param.

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