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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
The "main file" of the third repository, according to https://www.odoo.com/documentation/16.0/administration/install/install.html, seems to be https://github.com/odoo/odoo/blob/16.0/odoo-bin
For the first repository, gnucash, it appears to be a C++ application. The common convention for a C++ app is to have the "main()" function inside a source file with the same name as the library, e.g. "gnucash.cpp". And sure enough, here it is. If I didn't think to guess that it was in that file, I could always just download the repository and do a CTRL-F for "main(". That would lead me to it immediately.
A good example of such a repository would be Tailwind Elements: