Sharepoint Sizing

We are planning to design a sharepoint farm 2019 on premise to be configured as one drive on premise and to host 1000 users and to have 10 TB content database at day 1.

I am checking the Microsoft best practice when it comes to DB sizing... and I have few points that i would like to clarify:

usually if we have an always on group and when configuring the sharepoint i only have the option to select the DB server (Listener in our case) and all the DB will be created under the default location.

In the below article MS is recommending:

https://docs.microsoft.com/en-us/sharepoint/administration/storage-and-sql-server-capacity-planning-and-configuration

• Add another database server when you have more than four web servers that are running at capacity.

• Add another database server when your current server has reached its effective resource limits of RAM, CPU, disk IO throughput, disk capacity, or network throughput.

Question1: what is the added value of adding more DB servers they operate in active standby mode and we can have only one active server at a time since one availability group will be configured. Do we have other option?

 Regarding content database MS is recommending to :

• Only create files in the primary filegroup for the database.

• Distribute the files across separate disks.

• The number of data files should be less than or equal to the number of core CPUs. Count dual core processors as two CPUs for this purpose. Count each processor that supports hyper-threading as a single CPU.

Question2: Is there any official article how to achieve on the content database level?

Question3: what if we follow the standard way by having a disk of 15 TB and we create an AG including all SP DBs and we make sure to maintain 1 IOPS per GB, anyone tried something similar in large farms?