You are correct. So your overarching network would be 10.0.10.0/16 which has been subnetted to have a sub-network of 10.0.20.0/24. You will have a shit ton of subnet sizes available in you /16 but for this purpose if you wanted exclusively /24’s in your /16 your first would be 10.0.10.0/24 with a broadcast of 10.0.10.254. Your next network (10.0.20.0/24 the begins.

Now because these are individual networks you would then need to set up routing for them to be able to talk to each other. You can do this a number of ways, either a router in each network with a link in between all of them like a massive mesh network. Or the simpler way which is to have a single router in a 172.16.0.0 /24 network for example, with each interface uplink to your /24’s falling in each /24 subnet. So you would have something like this:

Router a : Router ip 172.60.0.1/24 Router to sw1 10.0.10.1/24 Router to sw 2 10.0.20.1/24

Switch 1: Switch 1 ip 10.0.10.2/24

Switch 2: Switch ip 10.0.20.2/24

Then on your router and switches you would would use a routing protocol such as ospf or eigrp to learn about neighbouring networks.