Here's a few quotes that might help.
Here's the First describing itself in Amends:
Hmm. You think you can fight me? I'm not a demon,
little girl. I am something that you can't even conceive. The First
Evil. Beyond sin, beyond death. I am the thing the darkness fears.
You'll never see me, but I am everywhere. Every being, every thought,
every drop of hate.
Here's Giles and Willow discussing the First in Bring on the Night:
WILLOW
But we haven't found any information on The First. No documentation"”
GILES
(crosses arms) That's because it predates any written history, and it rarely shows its true face.
Jasmine describes herself as a Power That Was - forged in the fires of creation. Here she is describing the origins of Demons, The PTB and our reality:
Yes. In the beginning, before the time of man, great beings walked the earth. Untold power emanated from all quarters the seeds of what would come to be known as good and evil. But the shadows stretched and became darkness, and the malevolent among us grew stronger. The earth became a demon realm. Those of us who had the will to resist left this place, but we remained ever-watchful.