Maximum memory for SQL Server 2000 TB when my RAM is just 512 GB
Think of the "Maximum server memory" setting in your right screenshot as a limit setting. You can configure SQL Server so that it won't use any more than a certain amount.

By default, this number is effectively unlimited. (That's what your server is set to.) It's kinda like setting your own personal financial budget - by default, many of us are willing to spend an unlimited amount of money on things that we should probably avoid.

Generally, you want to cap SQL Server's memory consumption, and there are varying recommendations out there. In our SQL Server Setup Checklist, we recommend leaving 10% free for the OS as a starting point. That means you'd set SQL Server's maximum server memory to 512 * 90% = 461GB, so you'd put 461000 in the max server memory box. (It's measured in MB, not GB.)

It might seem odd to leave over 50GB of memory free, but I've seen what you people do on your SQL Servers. You remote desktop in, you run SSMS, SSIS, SSRS, SSAS, and Outlook from the SQL Server. If you run a tight ship, and you never remote desktop in, then you can go higher on max memory. (Although since you're remote desktopping in to run this stuff, you should probably leave memory free. And also, stop remote desktopping in.)