If I record my own work using Siri, i.e. original texts, phrases, sentences, etc. who owns the copyright on those recordings?
Would it be the same as when using a DAW to record my own music?
More generally, it is the same as using any tool to create material in fixed form, which is protected by copyright. This applies to music, poetry and voice recorder with a tape-recorder, USB mic, digital recorder, or wax disk. Similarly, copyright is held by the "recorder" of written text put in fixed form using a pen, crayon, typewriter, Lanier word-processor or cellphone. If you write a book on an Apple computer, Apple does not hold the copyright to your book because they wrote the software that enabled you to write it.
However, if you use Siri's Dictation feature, you grant Apple and its subordinates permission to use your voice recording "to provide and improve Siri, Dictation, and dictation functionality in other Apple products and services". That does not mean that you grant them a license to sell your novel, but they may use your input for specified computational purposes.