Fibre ONT vs Modem

Hi all

I am doing a new build and going to use Ubquiti Products for reliabliity and want to seperate out the compents of the typircal "all in one" box to improve reliablility.

My street has fibre so the lines company (Chorus/Enable) will install an ONT.

I will install a Ubquiti POE Switch, Unifi Pro AP and a USG router.

Do I need the conventional ISP modem (by spark/skinny or whever I choose to go with as a provider)?

A router sorts the internal addresses doesn't it, and the USG has a firewall in it, so what does a modem between the ONT and Router actually do?

On a noraml ASDL connection I understand a modem converts the signal from the phone line into an ethernet, but on fibre doesn't the ONT do that?

The ONT just turns the fibre to ethernet, but do you need a modem in between to configure any ISP specific setting?

Thanks in advance :D