>>105593766
a) First, while a L3 switch can do many of the things that a router does, there are capabilities that it doesn't have. For instance, most L3 switches can't do GRE encapsulation for tunnels or IPSec/SSL VPN, and they also don't do DHCP.
Second, things like OPNsense aren't just routers either. They are more like firewalls, including stateful ACL application, IDS/IPS, and VPN capability.

b) Different ISPs set things up differently. Some like AT&T use 802.1X authentication, others use PPPoE. But some just use MAC whitelisting (whether it's explicit or based on DHCP reservations). And some don't do any authentication at all.