Audacity is used a lot beucase, supports windows and Linux, is free and open source. Was considered in some places a top 10 free open software. It have it's limitations you can't do music composition with it, is only of editing. Composition is for advanced users it require knowledge on music. Audacity is for mix, record, edit, applay efects. I did a couple of music mixes with it and the final result was pretty good.
Audacity does not have build in support for composition, but for most things that setting your project require this program is enough. You can also search for sampling music packagets, and use them on Audacity, for low quality composition.
Larger professional programs allow support to record all things from scratch some are even especialised for every type of task.
The online Digital library due to the high capability of music software and storage capability have grow a lot. You can get free music with average to high quality to be used in your project with out much efford. The quality is not top or AAA but for a Indie project can manage.