Following certainly is not a definitive answer. As you say, anything related to files and especially tracker is extremely obscure. However, it may help provide some insight. "Documentation" is installed with the Help function in your system. The section "Files, folders & search" informs about the use of Gnome Files.
Search syntax in Gnome Files without tracker enabled is dead simple. There are no wild cards. For file name searches, it will search for all strings you supply. Thus, "docker action" or "action docker" will find "Docker in action.pdf". That, at least, is how it currently works in 20.04. It is quite possible that in 18.04, you only can specify words in the order as they appear in the file name, i.e., "docker action" to find the file.
With tracker enabled, nautilus passes its search query to tracker. Tracker works based on strings extracted from file names and of file contents. It does handle wildcards.
? represents a single character and
* represents multiple characters. It will return results when a string occurs within the file name or file contents, and it uses an and logic: only items containing all strings provided are returned.
d*s for example, will pick up any file with
d[one or multiple characters]s in the file name or in the file's content, e.g. Downloads", "Documents", ...
d?s will pick upfiles with
d[any single character]s somewhere in the file name, e.g. '**D**e**s**ktop', Project description.txt`.
20.04 will also pick up
The Docker Book ***.pdf. in your last query. 18.04 does not. Be aware that it may take time before a file eventually is taken op and retrieved in a tracker search.