Probably depends on the ISP and what sort of ONT we're talking. My ISP gives me a separate ONT with an ethernet connection for the router. In OpenWRT I set up a PPPoE interface on top of a tagged VLAN with the appropriate credentials and it works pretty well, I can get a public IP or an entire IPv6 prefix depending on cretendials.

I honestly don't know why they bother with this PPPoE + VLAN setup on top of a modern fiber network, but it is what it is.

You could probably get an ONT in an SFP package, if you want to eliminate a separate box. The problem there is that ISPs tend to have an allowlist of permitted ONTs on the network. Some ONTs allow you to change the serial number so that may work in place of the ISP box.