I presume you are talking about products like the 74VHC4066 for instance. Datasheet here:
These devices are quite similar electrically to mechanical switches in many ways. They can pass current in either direction, and they do not significantly interfere with analog voltages. A regular CMOS digital gate will normally convert any analog voltage not right at the switching threshold and produce logic level outputs.
The same basic functionality can be obtained but with superior (for some purposes) specifications using things like two discrete MOSFETs wired back to back and their gates bias properly. A practical implementation could be made by connection the gates together of each device. Both sources would also be connect together. The drain pins would function as bidirectional input and output. Somehow you have to drive both gates, and one of the easiest and more versatile ways would be with a photovoltaic optical isolator. These devices differ from regular transistor output optical isolators in that they contain solar cells which actually output a small amount of useable power.