I asume this is related to BC Saas (cloud)?

This option is actually mentioned here Business Central Add-in for Outlook FAQ - Business Central | Microsoft Docs but it is not documented very well and I'm not sure it actually works. It is a little complicated as the solution seems to be to edit the xml files that are used for deployment of the add-in.

Now you don't need different add-ins for different companies within the same BC tenant, so I guess the idea is to have different add-ins for different BC tenants,

I think a problem is that you usually have a company in a tenant related to the companies domain (like CompanyName.com) ant the users are connected to the same domain, so a users Microsoft id/email/username is something like User.Name@CompanyName.com.

So the user opens outlook with an account for User.Name@CompanyName.com and when activating functionality in the add-in, then outlook will log into Business Central with this user.

Now maybe it would be nice if this same user could also use the add-in for an other company with domain OtherCompany.com, but User.Name@CompanyName.com is not a valid user in the BC/tenant for domain OtherCompany.com.

So maybe I don't exactly understand how this works, but it seems to be to be logically impossible, because the same userid (outlook account) cannot be a valid user in two different BC tenants (two different domains)?