Is it possible to submit to Nature Toolbox? Recently, I found this list of journals that accept articles describing open-source software. In the list, Nature Toolbox is mentioned. I looked it up and found that it is a category within Nature, but I have not found any information regarding the submission and publication process for this submission type.

Is it really possible to submit an article to Nature Toolbox or is this part commissioned-only?
"Toolbox" is not the name of a journal but of a section of Nature itself. As far as I know, articles published there are comments about existing software. It is  not possible to submit a "toolbox" article from Nature's manuscript tracking system, which most likely means that "toolbox" articles are invite-only.