Multisite sub-directory alias

I am getting very close and yet it feels unsolvable.
I am trying to create aliases for multisite sites. I installed wordpress in https://example.com/wp_blogs/, enabled multisite, and created two sites “first” and “second”. All fine so far. What I want is to make the add the following aliases:

https://example.com/b/first –> https://example.com/wp_blogs/first
https://example.com/b/second –> https://example.com/wp_blogs/second

I created the folders /b/first and /b/second, and added a simple .htaccess file inside that simply rewrites the url to /wp_blogs/*.
So far I’m getting only “Error establishing a database connection” errors when visiting /b/first/.

I can’t figure out at which point it’s failing. I don’t mind losing access to /wp_blogs/first/, if it redirects immediately to /b/first.

The reason I’m doing it this way is because there is another folder /b/third which contains a static HTML website, and I’d like it to use the same url (/b/*).

Is that possible? Is there a way to change the site url and make wordpress accept the alias as if it was the site url? Is there a better way to do this?