I'm using Wordpress multi sites with the domain mapping plugin. My websites used to run on sub domains the name looked like this example.test.com and the main site was test.com. With the domain mapping plugin I managed to turn sub domains into normal looking domains. my sub domain now looks like example1.com. My question now is: Do when I install an ssl certificate on which domain do I need to install this on, the sub domain or the domain I map to ?
Normal SSL certificates are for single domain or subdomain only. But, you can get Wildcard SSL, and that one SSL covers all subdomains for a single domain. Or get Let's Encrypt free SSL for each subdomain (if your host supports it).