I have an old Atari 800XE, so can I do demoscene on this if I want too?
You can do demos, slideshows, musicdisks, etc. on whatever hardware you please. The only "problem" is that not necessarily the hardware of your choice has a demoscene movement connected to it.
Anyway, I guess Atari machines do have an active demoscene.
And it's all done by code?
What do you mean? Code is just one of the key elements

saimo