Hi,
I meant how to measure how busy is SQL Server?
You need to be more focused. By "how busy is SQL Server" do you mean the CPU of the machine, Memory or maybe number of transactions per second or Maybe you want t monitor the IO
As a start point to get most of the information in a GUI interface, you can use Activity Monitor in SQL Server Management Studio
Or you can use the dashboard reports of the SQL Server Management Studio
https://www.sqlshack.com/how-to-collect-performance-and-system-information-in-sql-server/
For more details and using transact SQL queries we should know what exactly do you want to monitor, as I mentioned above