The interchange format includes a computer readable description of the FPGA which tells the tooling what is available inside the FPGA and how they can be connected.
If you are motivated by learning rather than practical usage, FPGAs are a great way to learn more about how things like CPUs or peripherals can be designed. It is also a way to understand how to evolve hardware and software at the same time.
Two examples that I have been loosely involved with;
* Google's CFU playground which is all about profiling and adding small number of new OpCodes to build an accelerator for a specific ML model (http://cfu-playground.rtfd.io/).
* The Fomu workshop (https://workshop.fomu.im) which walks though treating an FPGA like an embedded MCU where you can then modify the MCU!
That works, and has been done before. It can be confusing to run Speedify and the VPN client on the same box, but if there's a Speedify router box with the internet, and another box with the VPN client, it should just work.
On the actual compilation side with there are https://github.com/alainmarcel/Surelog and https://github.com/alainmarcel/UHDM which are then being coupled with open source tools like Yosys to allow targeting Xilinx 7 Series and Lattice ECP5 FPGA ICs with fully open source flows using fully open source FPGA tools like symbiflow.github.io
