Open-source chips is probably the difficult part about it. There's RISCV CPUs with open-source high-level description - but the physical devices are not open-source (for example the physical layout). There's some people running soft-CPUs on FPGAs. This way the circuit of the CPU is open-source but the FPGA fabric is not. I believe (and/or hope) that the situation might improve in the next decades. There's some chip fabs which start to allow creating fully open-source chips (Skywater in the US, IHP in EU). Yes, most chip fabs are actually forbidding to create open-source chips.
This might be inspiring: https://mntre.com/