The user account page is created when you activate memberships in Webflow. This page has a unique form, the user account form. Clearly the user account page and form must only be accessible to individual logged in users. Does this apply to other forms or data put onto the user account page?
The user account form can have custom fields added to it but it would be nicer to be able to use other forms to collect information and manage their data.
That is correct (mostly), the user account page is accessible to anyone, and everyone, who is logged into your memberships site.
The data that populates the form is called through API for that specific logged in user (no other users).
So the data is only accessible to that specifically logged in user, the form is displayed for all logged in users.
That’s why you typically see a slight delay when that page loads until that specific users information is populated in the form. It’s making an API request in the background for it.
Yea and no 
Only those who are logged into your membership site have access to the account page. But that also means everyone who is logged into your membership site has access to the account page.
The only thing that is specific to the user logged in is what’s returned from that API call. Meaning, any custom fields you add to the user account and specifically tell Webflow to return (via the Designer).
If you just drop any element on the account page, all logged in members will see it.
The best approach for this is to use the built-in CMS Collection Pages. You can associate a member to a CMS item and allow them to manage data there.
Scroll down a tad bit on this forum post and I talk about (at a high level) how to associate the CMS to Webflow Memberships.
There are other approaches, but you need to be careful with peoples info.
If you need to go beyond that Memberstack is really the best option until Webflow catches up, it shouldn’t be too bad switching back once they do.
If interested, here are a couple of screencasts walking through the quick Webflow + Memberstack setup:
How to create unique member dashboards in Webflow (using MemberStack)
How to create personalized content for logged in members with Webflow and Memberstack
Hope that helps!