Hi @lukehler
No.
No.
_redirects go into the directory that is published, there is no requirement that it has a specific name (e.g. public) and no requirement for a build command.
In _redirects format the above rule would read
http://www.custom-domain.tld/* https://destination.tld/path 301!
Have done this myself. As long as the formatting is correct no issues (that I have found).
Redirects are part of post-processing (post-build.)
Of course, as you don’t appear to wish to mention the domains you are using/testing it is impossible for anyone to test the behaviour for you. If you wish to have serene-austin-54062e.netlify.app redirected elsewhere, you will need to put in a rule for that too, otherwise a blank page will load.
If regardless of the custom domain visited (example.com, site.com, website.com, etc.) the destination is always the same (e.g. mywebsite.com/path) then you can have one rule to cover everything
/* https://detination.tld/path 301!
_redirects or netlify.toml
[[redirects]]
from = "/*"
to = "https://destination.tld/path"
status = 301
force = true
This would cover any custom domain as well and the <site>.netlify.app domain too.