Does publishing my code on GitHub make it FLOSS automatically? Is there some license automatically assigned to it? Is it "technically or say literally, open source"?
No, GitHub allows releases with different licenses. If you do not choose one, it is the usual basic assumption: all rights reserved. This is however overlayed by the terms of service you (implicitly or explicitly) accepted by using the site and creating an account. In the case of Github it allows others to view and fork your project:
However, by setting your pages to be viewed publicly, you agree to allow others to view your Content. By setting your repositories to be viewed publicly, you agree to allow others to view and fork your repositories.
Other websites may have other terms of services, and in a few cases an open license is automatically assigned, for example in Wikipedia and StackExchange user posts have automatically the CC-BY-SA license assigned.
GitHub has a help-site for choosing an open source license. Read there for more information.
At this help-site from Github is also noted, that your project doesn't have an automatic license applied but some things from the terms of service:
You're under no obligation to choose a license. It's your right not to include one with your code or project, but please be aware of the implications. Generally speaking, the absence of a license means that the default copyright laws apply. This means that you retain all rights to your source code and that nobody else may reproduce, distribute, or create derivative works from your work. This might not be what you intend.
Even if this is what you intend, if you publish your source code in a public repository on GitHub, you have accepted the Terms of Service which do allow other GitHub users some rights. Specifically, you allow others to view and fork your repository.
If you want to share your work with others, we strongly encourage you to include an open source license.
Unfortunately, many open source projects does not specify the license on github. Legally, anything that comes without a license that provides freedoms to the users defaults to all rights reserved. On GitHub, there are no defaults.
Language giving permission to "view and fork" in the GitHub TOS are 1) not a license; and 2) may have no legal effect. Do not confuse this with a real license.
Without a real FLOSS license associated with your project, it is not free software, and others will probably want to not use it because of the unknown legal hazards associated with doing so.
You decide what license to use. A rundown of the properties of each is outside the scope of this question. However, to be called "Open Source" or "Free Software", the license should preserve the four essential freedoms.
The FSF provides an excellent guide to free software licenses.
There is also a licensing FAQ at GitHub.