Yes, but genes in and of themselves explain very little in terms of the overall functioning of a cell. The process that determines if and when a gene gets expressed is probably as important as the gene itself.

Genes determine expression though.

Sometimes, but there is a lot outside the specific gene that has an impact.

Yes, other genes. Transcription factors. Methylation. DNA accessibility. All determined by proteins which are in-turn determined by genes. Genes determine expression.

What about all that DNA that never gets transcripted?

To do anything, requires interaction with a protein, which in turn is expressed by a gene.