Assuming the web root directory is under
and you need to redirect any sub-directory to the 'default' sub-directory, you would need an .htaccess rule like the following:
RewriteRule ^subdir.*$ /default/$1 [R=301,NC,L]
You can test your htaccess rule on a site like this one: http://htaccess.mwl.be/
What that site is doing, is checking the Regular Expression you have provided on the first block of the the RewriteRule, and the matched results will get redirected to the path you want to.
Keep in mind that the above rule might not always work, depending on your apache configuration.
You can play with different regex expression by testing the expression on sites like https://regex101.com/
Also a good place to learn about regex is https://www.regular-expressions.info/