DBurn the directory in which to run the commands is the directory that contains the composer.json, composer.lock, config.php and flarum (file without extension)
It should work the same whether you installed via Softaculous or the official installation guide. But I'm not sure whether Softaculous keeps the public folder or not.
You likely won't be able to install flagrow/ads because it isn't compatible with the latest Flarum version. There is a replacement extension available which you can find under the Extensions tag here.
There are multiple discussions around about the reasons to use Composer but basically using an existing package manager allows us to focus on the forum part and not have to dedicate our limited manpower to developing a custom updater and marketplace. This will come eventually, but we just don't have enough resources to create and maintain that at the moment.
