I have an extension which was installed in app/code and override from app/code, now I have only the zip for the composer installation, is it a way to install this extension by composer but in app/code/extensionsuppliername/extensionname instead of vendor/? Thank you in advance ps: in case of minus please let me know why so I can improve my post
You have two possible ways
#1: Please remove the app/code/extensionsuppliername/extensionname then add composer the composer entry and run the composer update, setup:upgrade
Note: If you go with #1 After removal of app/code/extensionsuppliername/extensionname, please run setup:upgrade. If no issues found then it will be fine.
#2: Download the module from zip and replace it in app/code/extensionsuppliername
I believe that is a normal scenario the plugin you are trying to install should have the folder structure, so it should automatically place it in app/code.
If you are trying to install it by hand you can always look inside a controller to see what the path should have been and build the route accordingly.
This seems like a good explanation of what I am trying to say : https://magently.com/blog/magento-2-the-right-way-to-install-external-modules/
You could achieve this by modifying the composer.json configuration file, more info here.