"Today there are specialized software to create demos but you can do it without these." It's a bit poorly formulated because back in the day people had nothing but a text editor.
I was going to mention Scream Tracker, but the Swedish language link names Fasttracker (apparently Swedish made, while Scream Tracker is by the Future Crew who hail from Finland).
There's been specialized tools in use since the very beginning though. I'd count assemblers and such among them. :)