I believe you need to use tweak the config/general.php file in order to have a baseUrl for a site that is a different domain name.
'siteUrl' => [
'english' => 'https://www.mysite.com/',
'japanese' => 'https://www. myjapanesesite.com/',
See documentation at: https://docs.craftcms.com/api/v3/craft-config-generalconfig.html#siteurl
I might be wrong on this but...
After running composer, did you have to go through an install process? If so, the DB/site you were given might have been configured to use a database table prefix.
If you look in your database, do you see some tables call assets then a second one called xxx_assets (xxx_ could be anything)?
If so, in your .env file, make sure ...