A subdomain is part of the domain configuration on DNS level. I’ll try to explain it without too much of my nerd jargon.
At your domain registrar, the company that leases the domain name mybusiness.com to you, the domain settings are kept in DNS records. These records describe how you want that domain to be used and which subdomains you want - and where they should point to.
So let’s say in your case you have [www.]business.com that is configured to go to the webhosting.
You might have [mail].business.com configured to handle the mail protocols
You might have [cms].business.com go to the cms login
And then there’s [intranet].business.com, which goes to the intranet (either via public router ip or any other way)
Now, if you publish a site in webflow, you are not touching those external dns records. You are merely publishing to the web hosting server, that is coupled to www.business.com.
There are a lot of nuances so they might have it setup differently, but the essence of my post is; that intranet subdomain is just a pointer in DNS records. It’s not a folder you can overwrite by publishing to a web server.
I don’t know if webflow takes over the DNS records when you go webflow for hosting, you should read up on that. But if they do take over DNS record authority, and for some reason not copy the records as-is, you should be able to add the intranet subdomain and point it back to where it was pointing in the first place. If this is the case, this could be found in the docs I reckon.
First step is for you to get a clear overview of the domain routing, dns records, where those are kept and check the webflow docs for info about how webflow ties into your existing infrastructure.