How to make www. and no www. Go to same site, for all sites?

Answered from this thread.

Summary:
The .htaccess redirect for www/non-www…

# www redirect
RewriteCond %{HTTP_HOST} ^subdomain.com
RewriteRule ^(.*)$ https://www.subdomain.com/$1 [R=301,L]

Put it ABOVE your WordPress rules on domain.com. Should work.

Example:

RewriteEngine On

# DOMAIN #1 - Redirect non-www urls to www
RewriteCond %{HTTP_HOST} ^domain1.com
RewriteRule ^(.*)$ https://www.domain1.com/$1 [R=301,L]

# DOMAIN #2 - Redirect non-www urls to www
RewriteCond %{HTTP_HOST} ^domain2.com
RewriteRule ^(.*)$ https://www.domain2.com/$1 [R=301,L]

# DOMAIN #3 ... ETC - Redirect non-www urls to www
RewriteCond %{HTTP_HOST} ^domain3.com
RewriteRule ^(.*)$ https://www.domain3.com/$1 [R=301,L]

RewriteBase /
RewriteRule ^index\.php$ - [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]

Marking topic as resolved. It would be nice if someone could explain how to force a trailing slash to each of my domains as well (if not present).