Welcome into the community.
To answer to your question:
do I have to create a subdomain on the server before creating a site
within the multisite admin dashboard, or after?
yes, you need to create subdomains on your server using cPanel or plesk.
if yes, do I have to install a WordPress installation in this
subdomain before creating a site within the multisite admin dashboard,
or after?
You don’t need to install wordpress for the subdomains.
Be careful to point the root of every created subdomain to the root of your wordpress multisite installation. Multisite network will share the same wordpress installation, so you don’t need to reinstall it each time you add a subdomain.
I had a similar issue, you can read the topic here.