OpenType fonts are indeed capable of much (much!) more than the old Adobe Type 1 format. Character encoding can be stored not only in byte format (which is the reason for the 256 glyph limit in Type 1 fonts) but as Unicode as well. This is not limited to 2-byte Unicode either. Fonts can, for example, contain emoji with Unicode code points well over
U+1FF00 (see Unicode.org's table for emoji).
Similarly, while ligatures are usually defined for two characters only, there is not really a practical limit to how many characters can be used. One of the best examples is the beautiful "Zapfino" ligature. Typing the font name (7 characters) will automatically show it as a single design:
Aside from basic ligatures, there are many, many more special features that can enhance an OpenType font. Whether or not each one of these work mainly depends on the software that the font is used in.
The specifications of OpenType fonts are publically available, but unless you are planning something extraordinary, I'd hesitate to recommend to try and "build an opentype font from scratch". The binary format is complicated, and lots of its parameters and the interactions with another are described only superficially.
There is not really "a step-by-step beginner's guide" to be found. By far most font designers use specific software to build their fonts with – and even then it takes some practice and/or trial and error to properly create and fill in all of the numerous tables to make the font work properly under all current software and OSes.
That said, if you are willing to read lots of documentation and not scared of command line utilities, then the Adobe Font Development Kit may be something for you to try. It comes with tools to convert .pfa and .pfb fonts to the OpenType format, add and test features, check the integrity of glyphs, and lots more.