Any idea on the margin question?
You have money in your account, and use it to buy stock. Now you have equity in the account, but no money. If you have a cash account, you can't buy any more. But if you have a margin account, the broker will actually lend you money (and charge you interest for doing so), so you can now buy more stock. They use the stock you already have as security on the loan, so if you stop paying interest they will just sell your stock for you and keep the money.
For example, suppose you fund your account with $10,000. You buy a nice blue-chip stock, say Dupont, with $9,000. (Many stocks have a standard "margin maintenance requirement", and Dow components all do. Tesla doesn't... it's volatile so they generally don't use the same percentage.) I think the margin maintenance for Dupont is 30% at my broker. Now your account has $1,000 cash, and $9,000 equity. Your "buying power (non margin)" is $1,000; you can spend this on stock, or withdraw it, and they won't charge you any interest. But your "buying power (margin)", if I understand correctly and haven't done the calculations wrong, is 70% of $9,000, or $6,300. You can use that to buy another blue-chip stock. (If you wanted to buy Tesla with it, you would only be able to buy somewhat less.)
Assume you do. At this point you will have used up your margin, and this is a dangerous place to be. Because if either stock reduces in value, you will fall below the margin maintenance requirement, and your broker will issue a "margin call", requiring you to top up the account, either by putting money in or selling some of your stock. Even worse, if nothing else changes, the interest on the loan will also force you into a margin call eventually. Unless your stock value is growing by more than the margin interest, this will eventually happen to you.
The worst thing about margin calls is that they usually end up making you sell something when you really didn't want to. So I personally don't ever go near the margin limit, and even when I use margin, I try to keep it short term, and eventually get back to something like 100% "equity percentage". But I don't mind buying something when an opportunity presents itself, and I can, because I have plenty of margin purchasing power.
Hope that helps.