Can't find a good side view of A & B units to determine what is between the A & B units and between the B unit and the first car just like diaphragms are between passenger cars. Maybe there is nothing between them.
On the E and F units there are diaphragms between the units plus MU, air, and steam, if passenger, hoses. Also both A and B units have a light for backing moves, normally on both ends of the B units. So in a passenger consist where the power was a F or E unit in an A-B configuration there would be a diaphragm between the B unit and the first passenger car. This is from the SOO line historical group. FP7-503 by John Moore posted Jan 13, 2019 at 6:37 PM
Diaphragms were not a universal feature on cab units. Many freight cab units were not equipped with them or had them removed later in their lives. As an example, here is B&M 4267, the last F-unit of any type to operate on that railroad and an engine I'm in the process of modeling. The door is open in the top picture. I believe the little horn above the backup light may also have been a B&M-exclusive feature. Of course, your prototype mileage may vary.
I can add to that also. Toward the end of their service life a lot of the B units were being run in lash-ups with newer power and could be found in the middle of a lash-up of GPs or SDs and the diaphragms were removed, and it also went for the A units. If they had been passenger units those not sold to Amtrak were stripped on their steam generators also and regeared for freight service. SP&S retained some Alco FB-2 units to splice into the middle of a couple of Alco C-636s because the Alcos were unreliable in the early years. And I have seen photos of GN SD units with 2 elderly B units spliced between them.
Originally diaphragms and drawbar couplings. As time went by, most converted to automatic couplers and diaphragms removed as they wore out or were damaged. Whether or not you use them depends upon era, and simply whether or not you wish to add that detail.
