working on watermarking

What are people referring to when they say, they are working on watermarking. Do they all develop their own techniques?

Yes. Most people who publish papers in this field are developing new algorithms. It may be helpful to think of watermarking "backwards", i.e. from the perspective of detection. Consider an image watermarking system.

Define some algorithm to "extract" a watermark (this could be taking the 1000 highest amplitude DCT coeffs, or averaging the 8x8 blocks of an image, or subtracting the original and projecting onto some subspace, or finding salient points, finding the Delaunay triangulation of those points and representing the result as a graph, etc.)

Modify the image so that the extracted photo watermark will be "similar" to some predefined watermark software(or set of watermarks). This may be done by adding something to the image or by multiplying the image by some spatially variant map. We may modify some values relative to others, increase one subset and decrease another subset of pixels or coefficients, warp the image to obtain a particular arrangement of salient points, etc. The modification might be done under the control of a perceptual model to limit the fidelity impact. It may be done under the control of a distortion model to maximize the robustness.