Presumably those problems are only affecting the Community container build, seeing as Online Distribution is offering an ARM64 of the non-Community variant:

NOTE: ARM Community containers are not available for this Developer Preview #6.

Why is this? The same happened for Preview #5.

Today ICR no longer offers build 589 of Community. It has been replaced by 599. I know this is preview, but I do wish InterSystems wouldn't delete the old build immediately they upload a later build. Those of us using dev containers or CI/CD workflows will suddenly find things broken until we update our scripts. And if we're only watching this DC post to learn about the new version we won't even have been notified yet.

Plus, still no arm64 build...

Still no ARM64 version of the Community Edition of this build?

(Asking for a friend; my employer hasn't bought me an M1 / M2 machine yet sad)

One of my colleagues just tried using the registration link but got this response from the Global Masters page it leads to:

For VS Code's native Search facility to search the server-side code your ISFS workspace gives you access to you must follow the instructions in the "Enable Proposed APIs" section of the extension's README (which is also available here). One of the steps will have to be repeated each time your InterSystems ObjectScript extension gets upgraded from MarketPlace with a new version.

my guess that if we specify port 57772 as the default port in the web server which I'm assuming that would be Apache , we don't have to specify the port in our url

No, if the web server (Apache or any other server) is listening on any port other than 80 (for http protocol) or 443 (for https protocol) your URL needs to tell the client app (e.g. your web browser) to connect to a port that is not the standard one for the specified protocol.