Temporary SSL certificates are simply Self-Signed certificates. It is no longer possible to create these in the panel, however there are two instances when you will see them displayed in your panel.
If you previously added a Self-Signed certificate in the panel
If you had added a Self-Signed certificate to your domain in the past, it will continue to function. However the SSL/TLS Certificates page now displays it as a 'Temporary' certificate. For example:
If you'd like to change this to a Let's Encrypt or Sectigo certificate, view the following article:
If you are in the process of adding a Sectigo certificate
View the following article for instructions on how to add a Sectigo certificate.
After its been purchased, the panel automatically installs it. During this process, a temporary certificate is set up.
When the Sectigo certificate is finally installed, it takes the place of this temporary certificate.