Started from zero six weeks ago and I don't understand what I actually own

It's real estate investing. The step you noticed is real and it does matter, so it's worth being clear about what changed.

When you own a building, you get rent, and you also get every decision and every problem. Roof, vacancy, the tenant who stops paying, refinancing, whether to sell. When you own shares in a REIT, a management team makes all of those decisions and you get a slice of what's left after they pay themselves and service the debt. The underlying cash still comes from tenants paying rent. You just have no vote on anything and no control over timing.

What you gained is real too. You can sell that position in ten seconds on a Tuesday. Try that with a duplex. And 2,000 dollars bought you a piece of cell towers and data centers and warehouses, none of which you could buy at that size any other way.

Cell towers and data centers count because a REIT is defined by what it does with income producing property, and long term leases on tower sites and server halls behave a lot like leases on a building. The rent just comes from a carrier instead of a family.

On the 21 dollars, reinvesting versus taking the cash is a question about your own tax situation and what account it's in, and that's a conversation for a tax professional. Mechanically, reinvesting buys more shares that then pay their own dividends. What most people trip on is that REIT dividends usually don't get the favorable tax treatment other stock dividends do, which is why you see people hold these in IRAs. Worth knowing what kind of account you opened.