Hotswap sockets, Mill-Max or solder

A hotswap keyboard lets you push switches in and pull them out without soldering. It does that with sockets, either Kailh hotswap sockets or Mill-Max sockets. A board with neither needs every switch soldered in.

Not to scale. Solder is in the accent color, sockets are outlined in it.

Kailh hotswap sockets are small plastic parts with two metal contacts, soldered to pads under the board. The switch pins go through the holes and into the socket. There is one shape for MX and one for Choc, and 6 designs have pads for both. Gateron also makes one for MX.
Mill-Max sockets are tiny metal sleeves that go in the switch holes. Keebio's guide: put one on each leg of a switch, push it into the board, solder the sleeves from underneath. After that the switch pulls out and the sleeves stay. Keebio says most Choc boards have holes too small for them.
Soldered is the simplest board with the fewest parts. Some boards take either a socket or solder, and the Hotswap line on the design page says so.

From each board's pads: 126 designs take Kailh sockets, 95 designs Gateron's, 170 designs Mill-Max, and 49 designs are solder only. For 4 designs the pads haven't been read yet.

Hotswap sockets, Mill-Max or solder

Takes Kailh hotswap sockets 126

Takes Mill-Max sockets 170

Solder only 49