hard to find conclusive information about gradle on-line
I don’t really agree. 
I find the user manual quite good for a software tool documentation. 
Blogs, SO answers, and so on are often a bit outdated or bad practice though, yes.
I actually also feel the on-line documentation is just incomplete.
In what regard?
The one-page version misses some parts due to reasons 
If you miss other information, it might be worth opening an improvement ticket.
I am aware of gradlew. i just misspoke, I meant to say gradle 8.8 is configured for my project.

what would be really helpful is a document,
That is hard to document, as it depends on what plugins you apply.
Each plugin you apply can add further configurations or link configurations together or also your build scripts can do that.
And some also are more an implementation detail.
So it would usually be up to the plugins you use to document which configurations they add, how they relate, and what to use.
There is for example The Java Plugin Figures 2 and 3 which show it for the java
plugin.
Or The Java Library Plugin for the java-library
plugin.
or a gradle command that lists the hierarchy of configurations
I’m not aware of one listing the hierarchy.
There is the task resolvableConfigurations
that at least shows the resolvable configurations and from which they extend from.
which ones are used in different gradle commands. for example, what configuration does the ‘gradlew build’ command use?
That is not really possible technically I think.
Especially the build
task does not use anything, it is a lifecycle task that just depends on other tasks.
Which tasks it depend on depends on the plugins you apply and what you configure in your build script.
But telling which task uses what configuration I don’t think is easily listable as things can for example also be mapped / transformed / etc., or depended on transitively.
Any source you can point me to?
You mean like this: Getting Started ?
If not, you probably need to ask more concrete about what you are interested in.
Is there a path to migrating a project from groovy to kotlin?
https://docs.gradle.org/current/userguide/migrating_from_groovy_to_kotlin_dsl.html
is it just a matter of renaming the build.gradle files and changing the code within to be compatible with kotlin?
Basically, yes.
What if you miss some files, would it be a problem to have a mixture of build.gradle build.gradle.kts files?
No, both can coexist without a problem.