Same way you do in survival mode, or press E and get obsidian from there.
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Nothing in vanilla aside from repeatedly hitting and creative mode can break obsidian. No amount of wither or TNT will break it.
Bedrock is indestructable under normal circumstances. It can't be destroyed by mining or by explosions, only removed in creative mode or by an admin. Obsidian on the other hand can't be damaged by explosions, but it can be mined.
No. Even though the player can destroy bedrock in Creative mode, it doesn't let dynamite destroy it. Blocks have a statistic to show how easily they can be destroyed by explosions. Cobblestone is the most resistant one that can still be destroyed by it. Above that is Obsidian and Bedrock, which can't be. This still applies in creative mode, which is why it can't be blasted away with TNT.
Download the mode or change the play mode to creative.
Yes, it is under "Materials" in the Creative Mode inventory.