when users upgrade let say for $32 plan, when making a page & choosing a domain we have to pay again aroun $16?
You can upgrade to any plan you want for your website. It has nothing to do with the domain.
You can buy the domain from any provider. ANY provider. It does not have to be Wix. Google “domain providers” to view a list of places that sell domains. There are cheap ones and expensive ones. ($1-$1,000’s)