I presume you're inquiring about non-traditional pathways? I bet you know you could attend a University apply for a research job with a lab or professor and go

I presume you're inquiring about non-traditional pathways? I bet you know you could attend a University apply for a research job with a lab or professor and go from there? You could always ask an important question, design several experiments[after researching experimental design], conduct the experiments, collect the data, see wether it confirms your hypothesis and what else it tells you, share that with the world in a meaningful(ideally machine parseable) good-looking way. Sure, most experiments in the categories you named may be infeasible do to costs and other constraints, but I bet a bunch aren't . . .