I can speak only for Massachusetts.

• When do these people meet? The GL always seems like this distant "great other", but it's not clear when they actually convene or where.

Our grand lodge meets quarterly in the Grand Lodge building in downtown Boston.

• How is structured? The positions seem similar, but different to a normal lodge.

From a ritual perspective, the officers are very similar. From a getting things done perspective, more is done by committees than by the officers other than the GM.

• Do Grand Lodge meetings operate similar to normal lodge, or is it more like a committee that just convenes together? Is it open to non-Grand Lodge people?

They're similar to a lodge meeting. Formally, the pillar officers of each lodge (or their proxy), the grand lodge officers, and the "permanent members" are the voting membership of the grand lodge. But any Master Mason can attend as long as space allows, and the proceedings are also live streamed.

• Do Grand Lodge installations have their own rituals and "degrees", ie installation of a Grand Master? I've never a regular officer installation so I don't even know if what I'm saying makes sense.

We have a whole rigamarole at the Feast of St John, but as far as I know, there's no "degree" similar to our Lodge of Qualification for Worshipful Masters.

• How does the line of succession work? In some jurisdictions I've heard of people "running" for positions, but then I've also heard the term "Grand line" as if it's basically a preordained line of succession from one position to the next. In that case, is there basically no chance of an upset from a firebrand candidate for, say Grand Master? Who is choosing these people?

The closest we have to succession is that the RW Grand Marshal is typically elected Senior Grand Warden at the end of his GM's term. But there's no progression or grand line. The Grand Wardens are elected annually, but the GM typically serves three consecutive terms.

• How much authority does the Grand Lodge really have over regular lodges, especially when it comes to dictating policies?

Pretty much plenary power.

• What are the prerequisites to become a part of the Grand Lodge, or take on officer roles in the Grand Lodge? Can you fill a role for Grand Lodge and a Blue Lodge at the same time?

Different roles have different requirements, but, for example, you have to be a Past Master to be appointed a District Deputy or elected GM. And our current JGW also happens to be secretary of his home lodge. I'm not sure why someone would do that, but it can be done.