Well,
If you want to do it yourself then go ahead. There are free programs, or I have been told that you can download into your computer such aa the Audicy Program.

You can buy Digital Recorders and Budget Microphones if you play an instrument and record that way.

You can Google Demo Services in a State and see what is avaliable or in Nashville, for instance.

I was using an old Reel to Reel Recorder, (Remember those?) until the one I was using died. Then I bought a Tascam 4 Track Cassette recorder and used a couple of Microphones I bought in the early 60's to record Guitar/Vocals. I still use it. I have a Stand Alone CD Recorder that I copy to for submissions.

Some use a computer and a Program to make demos. Whatever works.
You can spend loads of money on recording equipment or loads of money on Pro Demos. It is done every which way.

Ray E. Strode