Built my house for 60k (post-COVID dollars). A couple turns of 3" pipe into a tank. Up into a toilet. Not much more than that. It dumps into a big concrete tank and then from that the effluent goes out a couple pipes with holes drilled along it and into the soil. Quite clean and very cheap.

I think the 'plumbing' for sewage took me 2 days and $300 in pipe. The septic system was maybe $2000 in materials, you could easily replicate it with a shovel and a couple months of doing day labor on the weekends.

I take it for granted because it is extremely cheap, extremely easy, quite sanitary, and basically a footnote on building the shack. This ain't the 60s anymore, any tom/dick/harry can plumb a whole house using extremely forgiving pvc and pex with minimal cost or thought, though they're usually stopped by asinine licensing rules and the dumbass notion you need building plans (I had none).