@Metta: the Help file will tell you about the features of Obsidian, and about some but not about all the basics of markdown. For example colour is not an Obs-specific feature so will not be mentioned in the Help file.
To learn about markdown you can read up about on internet, there is plenty. BTW, markdown is not difficult to learn, and once you get the hang of it you’ll love it; at least that’s how it went for me. I ditched Word and other editors and only use MD.
Another issue is that MD is in fact so simple that not everything can be done using MD syntax, such as colouring text. Nevertheless, many (most/all ?) MD “flavours” accept some inline HTML syntax too, such as colouring text.
Adding colour to your text in Obs is done as follows:
<font style="color:red">bla text</font> , where “bla text” is your specific text.
It is also possible colour text and its background:
<font style="color:red;background-color:blue">bla text</font>
My advice is you should create a file as you are building your knowledge about MD and basic HTML, such as font colouring.
As for your 2nd question, by “commands” I presume you mean “syntax”, i.e. MD language/flvour. The flavour Obs uses is Common Markdown.
Formatting is mentioned in the Help file. In the Help file click on Start here at the bottom of the list in the left hand pane. On the page that opens, look under section Quick Start there is a bullet point How to use Markdown to Format your notes, with “Format your notes” being a clickable link.
I looked at and used a number of MD apps, and Obsidian is definitely the best overall, and still evolving, quickly. It is not complete yet, but it is extremely good already.
Embrace it, enjoy it, and feel free to ask any questions, the community is very helpful with many experts. You should also join the Discord channels.