Basically an API key is linked to an account. It inherits the access rights of that account. So if it only has read-only access to a certain network it will only be able to execute the GET calls pertaining to that network.
I just tested this and with an account that only has read-only access to a certain network I can execute this for that network:
{{baseUrl}}/networks/{{networkId}}/devices
But I can't do for example:
{{baseUrl}}/organizations/{{organizationId}}/deviceStatuses
{{baseUrl}}/organizations/{{organizationId}}/networks
So I guess it works as you should expect it to. You can execute the calls that do the same thing you would be able to do via dashboard with your account.