Multiple ways it can be done.
Most higher tier commercial/land mobile/public safety radios have a dedicated port on the rear. It's not just a Motorola thing.
These connectors will present various inputs and outputs that can be used. Various options include:
-data input
-data output
-audio out
-audio in/mic in
-detector output
-auxiliary inputs, used to trigger radio functions by external devices
-auxiliary outputs, used for having the radio trigger external devices
-switched power
-ground/earthing
Many of these connections can be customized via the radio software. Specific uses might include:
-data terminals
-credit card terminals
-GPS data for vehicle/user locations
-emergency function triggers/panic buttons
-"horn" alerts
-emergency lighting controls
-vehicle performance data
-external audio output to speakers/amplifiers
-external audio input from microphones, headsets
-radio programming interfaces
-some digital radios can also take camera inputs/video outputs with the proper interfaces
With relay input/outputs, you can use them to trigger external devices. We've used radios to trigger the lights in a fire station, PA system, alert signals, etc.
As for the physical interface, that varies.
Some Motorolas use standard DB25 connections, others use proprietary connector designs (16 or 20 pin connectors, 5 pin connectors, etc)
Kenwood uses DB25 or DB15 depending on the tier of radio.
Portable radios will use the side accessory connector used for speaker microphones and programming.
As for where to get the cables, the nice thing about the DB series connectors is that they are easy to find. The proprietary connectors, like the Motorola 16 and 20 pin connectors are sold by the manufacturer, but are not usually marketed to the end user.
You are not usually going to find off the shelf pre-made cables for commercial radios. These are usually custom fabricated by the radio shop who's doing the radio interface work. Since usage varies, each case will be different and producing "standardized" cables doesn't always make sense.
Software varies. Depends on what you are trying to do. There is no generic software for these systems. This is where the third party developers come in. They'll produce software and hardware that interfaces with the radio to do a specific task.
Some newer radios are using BlueTooth interfaces.
Where you might be confused is that you cannot think of this as a consumer product where you go to a store and buy a cable or software. That works with consumer electronics, but not for commercial radios.
Third party companies will often sell products through a vendor or manufacturer of a specific radio, or you can sometimes buy directly. Since these are specialized features designed to work with a specific family of radio, the market is designed for the radio dealers to sell this equipment/software as part of a turn-key system/product.
As an end user/consumer, you are going to have a difficult time finding this unless you get deep into the industry.
Amateur radio is a bit different. Since it's marketed towards the consumer, it's easier to find products. People will develop their own software or solutions for specific needs, sort of like the APRS or Packet radio systems.