Where can I ask questions and get support for other software (and Houdini in particular)?
williamatics For Houdini specifically it's hard to find free courses. According to SideFX you have a 10 day free trial on Pluralsight, maybe you can get some good start there:
https://www.sidefx.com/education/training-centers/
Otherwise the first result of "houdini courses youtube" search sent me on Surfaced Studio channel:
https://www.youtube.com/watch?v=pu_i6jeayLA
What other software did you have in mind ?
SideFX forums and od|force are the go-to places for questions regarding Houdini.
For learning, I suggest the Getting Started (especially the Foundations book), then doing Masterclasses (here, filtering in "All Levels" by "Masterclass"), along with reading the documentation (especially "Getting Started" and "Using Houdini" sections), which includes many examples and links to QuickStart videos for certain nodes and techniques.* These all are part of SideFX site.
A good third-party introductory course, and free, is the one by Rohan Dalvi.
*NB: The documentation is not complete everywhere, may be in some places out of date, etc., but, overall, is good enough. Something that works for me: if something in the docs seems off when trying to translate it to Houdini, I leave it for a while, learn other things, and when I get back to the problematic part, either I find that I was doing something wrong, or it becomes clear that the docs are outdated. Searching or asking in the forums also helps with sorting out these situations.
I was told this is a good resource for learning as well. http://www.appliedhoudini.com/