They are electrically and structurally interconnected. In other words, it is a certain set of active and passive components (resistors, transistors, capacitors, diodes, and so on) combined into a chip.
The chip is placed on a semiconductor substrate for subsequent signal conversion.
This approach makes it possible to abandon the outdated process, when for each task it was necessary to develop a signal conversion and processing scheme from scratch. Now you can use ready-made standardized solutions that have their undeniable advantages: