On the GDS-2000E line the front panel connector runs to a Spartan 6. The basic structure is one or two 500/1000 MSa/S dual channel ADCs, the Zyng, SPI memory, DRAM and the Spartan 6 which has connectors for the AWG and LA option modules in addition to the front panel and LCD.
The Siglent appears to be very similar from what Dave showed in his teardown. And logically, it's pretty much how you have to do it. Function pretty much forces everything to be very similar in structure, if not identical.
Communication with the front panel has to be done by polling a FIFO during a regular time slot in the acquisition cycle. If you used interrupts it could potentially disrupt the DMA. As @Marco noted previously, there is precious little DMA BW, so timing and sequencing everything is critical. A latency of 100 mS would be unnoticeable in the UI, but catastrophic in the data acquisition.
These things are running right at the edge of what the hardware can do. I'm extremely impressed by the HW designers, but very underwhelmed by the software engineering skills of the FW teams even at the top tier OEMs. Though at all the companies I suspect management and marketing are a significant impediment to the engineering.