How to get Brew formula from the filename of installed binary?
The brew whence external command takes an executable and looks up the Homebrew formula from where the executable comes.

Installing

To install the brew whence command, run:

$ brew tap claui/whence


Running

To run brew whence, pass it an executable:

$ brew whence python3



Executable                Comes from    
==========                ==========    
/usr/local/bin/python3    → python 3.7.7



Or pass it several executables at once:

$ brew whence 7z git-shell lzgrep



Executable                  Comes from
==========                  ==========
/usr/local/bin/7z           → p7zip 16.02_2
/usr/local/bin/git-shell    → git 2.26.0
/usr/local/bin/lzgrep       → xz 5.2.5



More information

For more information and examples, run brew help whence:

$ brew help whence


Full disclosure

Full disclosure: I wrote this command.
It is not part of Homebrew proper but you can inspect the code yourself.