Hi, if I understand your question, you have a multisite set up using subdirectories, so the URLs look like:
Root: https://www.example.com
Second site: https://www.example.com/site2
Third site: https://www.example.com/site3
Where site2, site3 and so forth are the names that you give the child sites.
Within WordPress you can change these so they would appear:
Root: https://www.example.com
Second site: site2.example.com
Third sitee: site3.example com
and so forth, with the subdomain being the name given the child site.
You can do this by going to your wp-config.php file and looking for the following line:
define( ‘SUBDOMAIN_INSTALL’, false);
Change it to:
define( ‘SUBDOMAIN_INSTALL’, true);
On any child sites you’ve created you may need to go to Settings > Permalinks, which will refresh the rewrite rules.
If you want to give the child sites their own separate domain names, though, such as:
Second site: https://www.somedomain.com
Third site: https://www.anotherdomain.com
You’ll need to purchase those domains and map them to the child sites. You don’t change anything in wp-config.php to do this. Instead, you need to install and set up a domain mapping plugin such as MU Domain Mapping (https://www.ads-software.com/plugins/wordpress-mu-domain-mapping/).