Hello again,
We have had another look at this and the answer to your question is that the response of /collections is paginated. As documented in
the endpoint's specifications, it only returns the first 100 collections per default. Unfortunately, ch.swisstopo.swissbuildings3d_2 is not among the first 100 collections.
You have two options to get to the ch.swisstopo.swissbuildings3d_2 collection:
1. Iterate over all pages via the "next" link in the "links" section of the response.
2. Restrict the result to the "swisstopo" provider like in curl
https://data.geo.admin.ch/api/stac/v1/collections?provider=swisstopo