I’m just looking for an explanation of what constitute “extremes”.

Centrist here. Far right is pretty easy and you've identified them. Far left can be seen in communities such as FuckCars. Another would be either recommendations for the government to build housing for everyone in city centers, taking over active (or inactive) real estate on the scale of millions. Elimination of all carbon-based fuels. Shuttering of the entire military. Since I'm a centrist I will tell you that those are fine ideals, just as educing regulations to the bare minimum of recommendations and balancing the national budget are excellent goals. None of them are practical because the leftist ones are either unworkable financially or would throw the economy into a black hole and the right ones are unworkable because there are way to many opportunistic motherfuckers out there that would poison their own mother to make an extra dollar or would throw the economy into a black hole.

I'm not in politics because I can't fucking afford to be. I would likely be a shoo-in for the local council. I was approached by the mayor and an existing councilman to do so. In researching it, I'd be looking at 500-700 hours a year of unpaid time. My neighbor is on the School Board. That's about 3/4 of the hours of Town Council, but it's a brutal cage match every meeting. For no reimbursement. Even at the state level, representatives are paid about $17k/yr and offers no benefits. That's less than 1/3 of what I'm paying to send my kid to college. Heck, my health insurance - with a $13k deductible - is $21k per year. And I'm just not corrupt enough to use a position like that to grift money on the side.

do-nothings, more focused on self-promotion than any kind of policy goal

Yeah, those aren't centrists. Those are narcissist's who don't otherwise have an extreme political (usually religious) agenda. A lack of political or societal ideals is not the same as a having centrist or moderate set of ideals.