i've used jd-gui.exe for years. there are probably others. a .jar is like a .zip file. you can use, eg, winrar to examine at least the top level (see attached image). you can't actually read the code (with jd-gui you can). just google "view .jar files" for something to download.
in the case of Core.jar, let's take this manifest editor comment: CreateResourceFromFile(Macro, Core.NetworkClearText)
this is like what chatgpt told you but missing the file that's used to create the macro: NetworkClearText.
if you look at Core.jar (ofr the image below), you will see NetworkClearText as part of Core.jar. in other words chatgpt had some of it right, but there are 2 issues: there is no file mentioned from which to create the macro, and even if there were, i don't think it would go in Core.jar. if you look at Core.jar with jd-gui, i don't think you'll find any class relating to passwords. and based on
@Erel's comment, there doesn't appear to be a b4a library to handle it.
whether someone has written a standaone library for autofill i can't say. there is an api, which means you could write a library or possibly use it with inline java. in any case, there is a separate file required to allow for autofill (you discovered this almost 2 years ago in android's documentation).
my purpose in responding to your thread was to point out that i believe chatgpt's code suggestion is wrong, at least as regards CreateResourceFromFile() is concerned.