With, for example FTTC, where a modem is provided for you, or with BT FTTP or a CityFibre service you would usually plug in a router and configure the router with your username and password. However, you can use a computer instead of a router, and get online by just plugging the network port of your computer in to the modem and set up a 'PPPoE' connection...

General Settings

PPPoE Service Name = Blank
PPPoE username = as supplied, e.g. x@a.1
PPPoE password = as supplied

Some services - eg CityFibre require a VLAN to be configured on the network card - not all computers will support tagging with a VLAN. eg, on windows, you may be lucky and see a VLAN ID option in the network adapter properties.

Windows 10/11

PPPoE Settings in Windows

Start -> Settings -> Network and internet
Dial-up ->Broadband PPPoE
Fill in the details of your connection with us
Click Connect

Mac

See: Apple

Ubuntu

See: Ubuntu