Let's say you have a 10 room concrete building. Let's say the concrete has rebar in it, so basically a wireless signal can't go between rooms. This is theoretical, just go with me.

If you wanted to have seamless wireless access across the entire house, so that you could take a voice call and walk around. You would need to put a wireless access point in each room.

In order to allow seamless transfer between access points, you would probably want to have access points from the same vendor, all configured using the same network ID, using the same authentication scheme. That way your smarter clients like modern cell phones, would see that oh there's multiple wireless access points in this network that I can talk to right now, I'll switch from one to the other. When the switch happens you tend to lose some packets, so the faster the switch, the more seamless the experiences for people.

Some wireless access points can negotiate with each other to push specific clients to different access points. But most of this fast switching is done client side.

so in this 10-room building, you're going to have 10 access points, you're going to have a single router between the building and the internet, you're going to have a DHCP server internally usually. Those access points may be negotiating between themselves, using some mesh technology, but ideally they're all wired to the same network.

When a client switches from access point a to access point b, those access points will now know that physical hardware address has switched, and the main network should still send the packets to the client with very few being dropped.