I have custom v7 modules under suitecrm/custom/modules/<my_module>.
After upgrading to v8, I placed them at suitecrm/public/legacy/custom/modules/<my_module>, but the v8 module loader does not recognize them.
Please add explicit guidance to the docs. I’m happy to contribute a PR once I have the correct process.
If you upgraded (using the SuiteCRM upgrade script) you shouldn’t need to do any of those manual copies, I believe.
I don’t think the Module Loader simply “picks up” modules from a directory; I think you have to install them, and it will likely save some references in database tables, metadata, I’m not sure.
But I would expect that modules installed in v7 would live through the upgrade process without further intervention.
If it didn’t come over via upgrade. You might want to try and export the module from SuiteCRM 7 and Import the module through the module installer. I’ve never done this (between 7 and 8) so take it as a suggestion. You may/maynot have to designate the module as a legacy module as well.
Generally, you can’t just copy folders from one install to another. The pereferred method is to export the module in the module builder and then install it on another installation.