A modem is a sort of “adapter” between physical mediums and protocols and sometimes also a router. It speaks DSL, fibre, cable etc. on one end and Ethernet on the other.

A wireless access point is similar in that is also is an “adapter” between mediums but it’s an adapter between physical and wireless. It effectively connects wireless devices to your physical Ethernet network (allowing communication in both directions) and never does any routing.

What you are typically provided by an ISP is an all-in one box that contains modem, router, switch, firewall, wireless access point, DHCP server, DNS resolver and more things in one device. For a home network, I wouldn’t want most of these to be separate devices either but at least wireless should be separate because the point of connection for the modem is likely not the location where you need the WiFi signal the most.