There is something I don't understand when configuring VPCs and subnets for aws...

This is the VPC configuration with 10.0.0.0 to 65536 network space (from 4th octet?) and
Is there no problem recognizing that subnets range from 10.1.0.0 to 10.1.0.256?

Subnets range from 10.1.0.0 to 10.1.0.255

If I wanted to start any instances on this subnet, would it be an image that allocates IP addresses in the range 10.1.0.0 to 10.1.0.256?

0,255 is reserved for networks and broadcasts. AWS also has 1,2,3 reservations.
I can use 251 pieces from 4 to 254.

If I run out of IP or want to separate subnets, is it okay to assign them in the same way as 10.2.0.0/24, 10.3.0.0/24, etc.?

No problem

Why do I assign VPCs at 10.0.0.0/16?

http://www.faqs.org/rfcs/rfc1918.html determines the space available for private addresses

10.0.0.0-10.255.255.255 (10/8 prefix)
 172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
 192.168.0.0 - 192.168.255.255 (192.168/16 prefix)

If you use something other than this, you may use it as a global address, so it is not impossible to use it. If you wear it, you will have to choose which route to go to.

I don't know why I use 10.
Starting with 10 is 24bit 10...*You can use it, so if you want 16bit, you should use 172
If I have to say something, 10 is easier to remember, so if you don't think about exhaustion, 10 might be fine.

I created a VPC at 10.0.0.0/16, and a subnet at 10.1.0.0/24.

The subnet is out of range for VPC.
Split the subnet within the address range of the VPC.

• VPCs: 10.0.0.0/16 ... from 10.0.0.0 to 10.0.255.255
  • Subnet1:10.0.0.0/24...10.0.0.0 to 10.0.0.255
  • Subnet2:10.0.1.0/24 ... from 10.0.1.0 to 10.0.1.255
    ::
  • Create up to Subnet256 (actually limited to 200 AWS)

• Subnet1:10.0.0.0/24...10.0.0.0 to 10.0.0.255
• Subnet2:10.0.1.0/24 ... from 10.0.1.0 to 10.0.1.255
  ::
• Create up to Subnet256 (actually limited to 200 AWS)

As shown in the picture below, there is a subnet in the VPC box and a server in the subnet box.

◆ VPC Image Diagram

Why do I assign VPCs at 10.0.0.0/16?

You can change the address range freely, so it depends on the design.
However, private address space is recommended.

• 10.0.0.0-10.255.255.255 (10/8 prefix)
• 172.16.0.0 - 172.16.255.255 (172.16/12 prefix)
• 192.168.0.0-192.168.255.255 (192.168/16 prefix)

For more information, see the User Guide.
https://docs.aws.amazon.com/ja_jp/vpc/latest/userguide/VPC_Subnets.html

(By the way) As you are studying networking, please note that there are some differences between actual networking and AWS.AWS is more abstract and easier.(That's the good thing)

Personally, I feel that using VyOS in a Docker or KVM virtual environment is more like a real network.If you are interested, please check it out.