Some of it was released by national governments, e.g. the United States, in the post-Cold War. The US did this as part of an attempt to "come clean" about the various side-effects of nuclear testing (part of the short-lived "Openness" initiative of the early Clinton years). Sometimes they were released even at the time, e.g. the US usually announced Soviet tests as a way of showing that they knew what was going on, that they were in control, etc.

Some of it is just a record of information that was collected by remote monitoring stations.

The thing is, it's not all that hard to tell when somebody sets off a nuclear weapon. It's not usually that hard to figure out roughly how explosive it was, or where it was set off. They make massive signals in the environment, be they seismic, acoustic, or radioactive.

For this reason, it is also the case that lots of countries don't mind announcing them. They sometimes leave the precise yields and timing out, but again, that is something that can be inferred with remote stations. So the question becomes, who are you hiding them from if you try to keep them secret?