Home DIYer here.
What type of connector is this, and how do I remove it?
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That right there is a crimp connector. Few ways to remove one, mostly you want to avoid damaging wire and losing length. It's a compressed ring.
First thing I'd try is side cutters. Cut between the wires and once one side of the ring is removed you should be able to bend it out of the way. That one looks particularly easy to cut this way.
If side cutters don't work, I'd evaluate whether I can afford to lose wire length. If it's not an issue, cut the wires where they enter the crimp. If it is an issue, I'd use a rotary tool with a thin disc to make the necessary cuts with minimal risk. If you put any significant gouge in the copper, it should be cut back to the gouge so you're only using good wire.