(Genuine question, I've been trying to find reliable, well documented, robust patterns for doing this for years! I need it across macOS and Linux and ideally Windows too. Preferably without having to run anything as root.)
https://danwalsh.livejournal.com/28545.html
One might have different profiles with different permissions. A network service usually wouldn't need your hone directory while a personal utility might not need networking.
Also, that concept could be mixed with subprocess-style sandboxing. The two processes, main and sandboxed, might have different policies. The sandboxed one can only talk to main process over a specific channel. Nothing else. People usually also meter their CPU, RAM, etc.
INTEGRITY RTOS had language-specific runtimes, esp Ada and Java, that ran directly on the microkernel. A POSIX app or Linux VM could run side by side with it. Then, some middleware for inter-process communication let them talk to each other.
https://github.com/microsoft/litebox might somehow allow it too if a tool can be built on top of it, but there is no documentation.
I trust Firecracker more because it was built by AWS specifically to sandbox Lambdas, but it doesn't work on macOS and is pretty fiddly to run on Linux.
Gleaned from https://github.com/containers/bubblewrap/blob/0c408e156b12dd... and https://github.com/containers/bubblewrap/tree/0c408e156b12dd...
It's not industrial-grade safety for public use, but it'll do for personal use. Other tools for it are also mentioned.