Is there a free and high quality application that offers editing, mixing, synthesiser and the whole range of tools useful for music creation? A sort of Blender for sound?
The "Blender for sound" is called Ardour [0]. I use it to produce all the content on this site [1], including all the music and sound effects you can hear.
Almost free. https://reaper.fm It's cheap enough for almost anyone to buy and you can play around with the free version.
This might not be the answer you were looking for.
Garage Band is free for iPhones, iPads, and Macs.
Maybe you already have an iPhone. Maybe you don't. But you have to buy hardware irrespective of what software you use.
Good luck.
Apple's Garage Band is free and pretty rich in features. There are probably many good open-source options too.
Audacity, but it's more for editing not music production.
> tools useful for music creation ... sort of Blender for sound
This is a very wide range of tools. Music can be represented and created in various forms. An established and widespread method is the presentation and processing of music in the form of individual events, organized in tracks and chronological sections. Famous commercial "computer aided design" tools for this approach are, for example, Logic Pro from Apple or Cubase from Steinberg. There are many open source alternatives; a useful list is e.g. here: https://alternativeto.net/software/logic-pro/?license=openso...
But music can also be represented and composed in the form of program code, usind a special programming language, for example. Here is a good overview of available (open source) solutions: https://en.wikipedia.org/wiki/List_of_audio_programming_lang...