dyld VS qubes-mirage-firewall

None of what? In case I understood you by chance, there's a whole darwin stack on github, grouped neatly at https://opensource.apple.com/releases/. There is a lot of argument about what's open source and what's not, I consider permissive licenses as open source. I did never claim that any Apple software is open source, but due to OS's modular nature I'm willing to say that macOS and iOS consequently is largely open source

A lot of Apple software including the operating systems is open source, though in practice many of the key components that you might need to solve a problem or understand a bug are missing from these repositories.

2) https://github.com/mirage/qubes-mirage-firewall is by far a better firewall for Qubes than OpenBSD ever will be - unikernels are far more secure than a traditional operating system is and you can read all about it on https://mirageos.org/

That's correct. It does mean that the closest to a self-contained program you can run is a unikernel like the mirage-firewall, unfortunately. On the upside, those remain easily portable to essentially anything that can run VMs so long as you adjust the image format.

That sounds similar to a unikernel. There are actual uses for those in seL4 and Qubes OS such as a firewall-qube (in theory unikernel qubes should be able to take far less system resources to run than full Linux+distro qubes).

Here's one that is "production" ready: the Mirage-Firewall microkernel running on Qubes OS.[0]

[0] : https://github.com/mirage/qubes-mirage-firewall

sys-net, sys-firewall and other administrative vms should slowly migrate to unikernels instead of running linux, which should help with ram usage. The mirage.io project seems to build a couple qubes vms, for example https://github.com/mirage/qubes-mirage-firewall is a firewall which they indicate to give 64Mb of ram.