Theme: Organization of work of local networks. Networking by domain structure. Contents

What if the sending or the target device is connected directly to the router without a switch?

In cases where one or both of the devices are connected directly to a router without the benefit of a physical switch, the logical flow of the message packets are fundamentally the same as described in the sample.  Just think of the router serving both roles, providing both the switch and the router functionality.

What if the sending or target device is communicating via a wireless access point?

While the physical differences between wired and WiFi connections are obviously very different, the logical flow of the message packets are fundamentally the same as described in the sample as well.  Just as in case described in the previous paragraph, the wireless access point provides both the switch and router functionality.

Conclusion

Each router port is configured with a specific routing protocol that is associated with that port's function.  For example, a router port that connects to the Internet must learn how to efficiently route communication messages to destinations around the world.  Protocols that facilitate this are called "gateway routing protocols" and have names such as the Border Gateway Protocol ("BGP") or Exterior Gateway Protocol ("EGP").  A router port that connects to an organization's internal networks must learn the how the organization's network is configured to efficiently route traffic throughout the organization.  Protocols that serve this purpose are called "interior routing protocols" and have names such as Enhanced Interior Gateway Routing Protocol ("EIGRP"), Interior Gateway Routing Protocol ("IGRP"), Open Shortest Path First ("OSPF"), Routing Information Protocol I and II ("RIP"/"RIP II"). 

References

1. 
    Compare: Anderson, Howard; Yull, Sharon; Hellingsworth, Bruce (2001). Higher National Computing (2 ed.). Oxford: Routledge (published 2004). p. 260. ISBN 9781136398988. Retrieved 2015-08-18. A network domain is more formally defined as a group for servers controlled by a primary domain controller. The idea is that this group of servers can behave as a single combined unit.
   
2. 
   ^ Compare: Chen, Lidong; Gong, Guang (2012). Communication System Security. Chapman & Hall/CRC Cryptography and Network Security Series. CRC Press. p. 313. ISBN 9781439840368. Retrieved 2015-08-18. The terminology, network domain, comes from the cellular systems. Traditionally, a cellular service provider owns not only radio frequency spectrums but also certain network infrastructure, for example, base stations, switches, and servers. All these entities are connected through wired network to provide telephony service. A network domain is the wired portion of an operator's network.
   
3. 
   ^ Postel, J (October 1984). "RFC: 920 - Domain Requirements". Internet Engineering Task Force. Retrieved 27 January 2019.
   
4. 
   ^ Archiveddocs. "Planning DNS Servers". docs.microsoft.com. Retrieved 2019-01-27.