FWIW, when I put <<linux create file image encrypted file system>> into Google (this was the first thing I tried, though without knowledge that it might be a tricky case I might have been less careful picking keywords) I get what look like plausible results.
At the top there's a "featured snippet" from opensource.com, allegedly from 2021, that begins with: create an empty file (this turns out to mean a file of given size with no useful data in it, not a size-0 file), then make a LUKS volume using cryptsetup, etc.
First actual search result is a question on Ask Ubuntu (the Stack Exchange site dedicated to Ubuntu) headed "How do I create an encrypted filesystem inside a file?" which unless I'm confused is at least the correct question. Top answer there (from 2017) looks plausible and seems to be describing the same steps as the "featured snippet". A couple of other links to Ask Ubuntu are given below that one but they seem worse.
Next search result is a Reddit thread that describes how to do something different but possibly still of interest to someone who wants to do the thing you describe.
Next search result is a question on unix.stackexchange.com that turns out to be about something different; under it are other results from the same site, the first of which has a cryptsetup-based recipe that seems similar to the other plausible ones mentioned above.
Further search results continue to have a good density of plausible-looking answers to essentially the intended question.
This all seems fairly satisfactory assuming the specific answers don't turn out to be garbage, which doesn't look very likely; it seems like Google has done a decent job here. It doesn't specifically turn up the LUKS manual, but then that wasn't the question you actually asked.
Having done that search to find that the relevant command seems to be cryptsetup and the underlying facility is called LUKS, searches for <<cryptsetup manual>> and <<luks documentation>> (again, the first search terms that came to mind) look to me like they find the right things.
(Google isn't my first-choice search engine at present; DuckDuckGo provides similar results in all these cases.)
I am not taking any sides on the broader question of whether in general Google can give good search results if one picks the right words for it, but in this particular case it seems OK.
At the top there's a "featured snippet" from opensource.com, allegedly from 2021, that begins with: create an empty file (this turns out to mean a file of given size with no useful data in it, not a size-0 file), then make a LUKS volume using cryptsetup, etc.
First actual search result is a question on Ask Ubuntu (the Stack Exchange site dedicated to Ubuntu) headed "How do I create an encrypted filesystem inside a file?" which unless I'm confused is at least the correct question. Top answer there (from 2017) looks plausible and seems to be describing the same steps as the "featured snippet". A couple of other links to Ask Ubuntu are given below that one but they seem worse.
Next search result is a Reddit thread that describes how to do something different but possibly still of interest to someone who wants to do the thing you describe.
Next search result is a question on unix.stackexchange.com that turns out to be about something different; under it are other results from the same site, the first of which has a cryptsetup-based recipe that seems similar to the other plausible ones mentioned above.
Further search results continue to have a good density of plausible-looking answers to essentially the intended question.
This all seems fairly satisfactory assuming the specific answers don't turn out to be garbage, which doesn't look very likely; it seems like Google has done a decent job here. It doesn't specifically turn up the LUKS manual, but then that wasn't the question you actually asked.
Having done that search to find that the relevant command seems to be cryptsetup and the underlying facility is called LUKS, searches for <<cryptsetup manual>> and <<luks documentation>> (again, the first search terms that came to mind) look to me like they find the right things.
(Google isn't my first-choice search engine at present; DuckDuckGo provides similar results in all these cases.)
I am not taking any sides on the broader question of whether in general Google can give good search results if one picks the right words for it, but in this particular case it seems OK.