Answers:
1.
The cluster default provider will be installed if no action is taken.
To install any other provider the user would have to uncheck the checkbox:
"Use cluster's default network provider"
this will activate the:
"External Network Provider"
dropdown and allow the user to choose another one
2.
Yes
If not selected, you will just create a regular ovirt network.
If the checkbox is not checked, the drop-down has no effect, even if it actually show anything (it's grayed out to indicate it's not relevant)
3.
You can use the one you already have
If you have installed it prior to 4.2, you will need to make a manual update to its "networking plugin" value (set it to "oVirt Network Provider for OVN").
It is likely for customers which have installed for trial in 4.1