I did not watch that vid but it sounds like it’s for moving single sites, not multisite “sites”. You can develop a child theme on any other site and then activate it on the main site once it’s ready. Site content can be moved by simply renaming tables in phpMyAdmin. Rename the current main site tables you need to replace to a non-WP name like “backup_posts” (from “wp_posts” or similar, not all main site tables are to be renamed). Sub site tables are typically “wp_3_posts”, etc. Rename those tables to the proper equivalent main site table (“wp_posts”). Do not do so for the options table though, there are important settings in the main site table that do not exist in sub site tables. This means customizer settings among other things will not be copied over. While you could copy over individual option values as needed, it’s safer to simply set the desired customizer and other options as needed through the site’s back end.
Messing with the DB with phpMyAdmin is inherently risky business. While just renaming tables is relatively safe and easy to revert, please export a full backup of the WP DB before attempting any changes.
You must replace the sub site tables you renamed with something equivalent. Either duplicate tables from a newly created, nearly empty sub site, or rename the original main site tables from “backup_posts” to “wp_3_posts” or whatever is applicable to your situation.
It’s normal for multisite to hide the overall site and home URL settings. You can access them through the hidden /wp-admin/options.php admin screen. All of the hundreds of options are listed alphabetically, though it’s risky to change some values through this page. Changing the main site’s URL is more involved than simply changing these two settings.
