There's four great paths for this:
1. Purchase land in the forest and clean it up. Contrary to popular opinion, you can cut down almost anything you want down to clear cutting if your heart desires. There are special rules around oaks, but you will not find many (realistically: any) oaks in wildfire prone areas. There are numerous grants available for private landowners to build a more resilient landscape and replant with diverse, fire-hardy species.
2. Join the USFS or CalFire where you will do this work on public lands. All fall/winter/spring most of the effort is spent around cleaning out undergrowth, burning piles, doing controlled burns, and clearing out standing dead.
3. Join the California Conservation Corps https://ccc.ca.gov/
where they will put a chainsaw on your back and send you out to the forest to clean up the forest.
4. Get firewood / Christmas tree permits and harvest your firewood from public lands. There's no rules that say you can't clear out standing dead and smaller trees.
From my own personal experience, I might suggest that one man with a woodchipper is not the right mindset to strive for. A woodchipper may be able to clean up maybe 10 acres a year (I'd guess closer to 5), assuming full time work in a low elevation (no snow) landscape, plentiful funds for gasoline, and mechanical expertise. Wood chippers don't travel well in the forest. But one man with a chainsaw and a can of diesel can clean up hundreds of acres per year with burn piles with similar effort.
There is plentiful motivation from the USFS and California government to clean up their forests. But the scale and terrain of our forests escape human imagination. Thirty three million acres of granite cliffs, flooded valleys, and high elevation bowls with hundreds of feet of standing snow in the winter. If you're passionate about this, I'd suggest to start small and local. Volunteering for CalFire is a great way to get more expertise in the nature and behavior of our wildfire ecosystem.