Multisite and giving wordpress its own directory

.htaccess

<IfModule mod_rewrite.c>
# not localhost:
# RewriteCond %{HTTP_HOST} \.
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
#eliminate root path access denied: (without placeholder index.php)
RewriteRule ^(/)?$ wp/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).*) wp/$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ wp/$2 [L]
RewriteRule . wp/index.php [L]
</IfModule>

Note: Newly created multisite sites needs default themes (for example: twentytwenty) for initialization.