You are confusing blocklists (adlists) with blacklists.
Adlists are URLs that lead to a text file with domains to be blocked. When you subscribe to such a list (by adding the URL in Group Management > Adlists), when you update gravity Pi-hole opens the URL and retrieves all the domains, and stores them in the gravity database. Gravity is updated manually (as you noted) and weekly on Sunday morning via a cron script installed by Pi-hole.
If you want to add domains to your blacklist, the process is similar but you add the domains directly (and don't use a URL), and these are not updated weekly (they are local to your Pi-hole). This is done in Group Management > Domains. You can add a number of domains at once - copy the lot to your clipboard and paste the clipboard contents to the domain entry window, and select "Add to Blacklist"
You will have very little success. YouTube ads are served from the same domains as the content, which makes them difficult to effectively block with a domain blocker (i.e. Pi-hole). Many of the blacklist entries you will find will also block videos or other features of YouTube.