Select the object then look for the "UV Editing" workspace tab at the top. Press "A" to select all geometry. We want to see the UVs that'll appear on the left. In case there's no image under the UV you can select it from the header in the UV Editor or click on it from a Shader Editor and it'll appear automatically, but in…
Well, while breakdowns and explanations about wavy bakes are indeed useful, the artefacts in the work of the OP aren't caused by that. They are more than likely caused by the baked pixels of the normalmap instantly jumping from one value to another across the (hard) edge but without UV room to breathe since the UVs have…