I've spent a lifetime in software development so I'm very familiar with programming, etc. And in the "old days" when it was just VORs and ILS it didn't really matter how all the parts fitted together because all the representations were standard. So a NARCO VOR indicator looked the same as a King and either would drive an autopilot.
But today it's not obvious to me what components are responsible for what functions and how they communicate with each other.
The fact that Garmin G3X Touch will only interface with Garmin navigators (according to Garmin) indicates that these interfaces are not standard. I watch a lot of YouTube videos and one of my favorites is Kevin (N771BC) as 310 Pilot. His airplane is equipped with Avidynes interfacing to an STEC 3100 autopilot which relies on 2 G5s. But the 3100 cannot drive the FD bars on the G5 but the GFC500 of GFC600 AP can. Again, this indicates that interfaces are not standard.
What I am looking for is a good book, or PDF or article that explains how all these modern avionics work together, or more precisely, why they don't work together. If anyone can point me in that direction, I'd be very grateful.
Most of the old interfaces are still there, and semi-standardised, but there are new interfaces on top of them, and emulators on top of others, and some of those are proprietary.
In my simple Warrior II, my single-axis STEC-20 autopilot takes a heading-deviation signal from my mechanical DG (with heading bug) for when it's operating in HDG mode, and a course-deviation signal from one of two mechanical CDIs for when it's operating in HI or LO nav mode. So far, these are all 1970s-era interfaces.
There are catches, however. I have a switch between two mechanical CDIs for the autopilot, and one of them is driven by my GTN 650, which sends digital (rather than analogue) course-deviation signals to the CDI, but also a mode indication signal (RNAV vs VLOC). This is still pretty standardised, but it's newer (GNC 430-era stuff, about 20 years old). The GTN 650 also sends GPS steering commands (standardised over a digital RS 232 interface) to an STEC GPSS unit that sits between my DG and autopilot. When I switch it from Heading to GPSS mode, it replaces the heading-deviation signal from my DG with a simulated heading-deviation signal based on the roll-steering commands from the GPS.
So, even in my simple plane with rather basic avionics, I have a lot of input paths leading to my bare-bones single-axis autopilot.
1. Roll knob -> A/P (roll mode): hold the bank angle set by the knob
2. DG (heading bug) -> GPSS (heading mode) -> A/P (HDG mode): steer to the heading bug on the DG.
3. GTN 650 (RNAV mode) -> GPSS (GPSS mode) -> A/P (HDG mode): take steering commands from the GPS (can even fly me around a hold, which is sweet)
4. GTN 650 (RNAV mode)->CDI1->A/P source switch (NAV1)->A/P (HI nav mode): track an RNAV course with high sensitivity
5. GTN 650 (VLOC mode)->CDI1->A/P source switch (NAV1)->A/P (HI nav mode): track a VOR or LOC course with high sensitivity
6. KX-155->CDI2->A/P source switch (NAV2)->A/P (HI nav mode): track a VOR or LOC course from my nav2 radio with high sensitivity
7. GTN 650 (RNAV mode)->CDI1->A/P source switch (NAV1)->A/P (LO nav mode): track an RNAV course with low sensitivity
8. GTN 650 (VLOC mode)->CDI1->A/P source switch (NAV1)->A/P (LO nav mode): track a VOR or LOC course with low sensitivity
9. KX-155->CDI2->A/P source switch (NAV2 position)->A/P (LO nav mode): track a VOR or LOC course from my nav2 with low sensitivity
There are lots of switches/indicators to check to make sure my A/P is really taking input from where I want it to. Every plane will be set up a little differently, so there's a learning curve. If I had fancier stuff, like a G5, that's when the proprietary interfaces would kick in (though it still uses a lot of de-facto standard interfaces).
This publication is a bit out of date -- it's from 10+ years ago, and a lot has changed in that time -- but I still wish I'd known about it when I was first trying to learn about all this stuff, when I installed my GTN 650 back in 2017:
https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/advanced_avionics_handbook/