To be clear, CRISPR-Cas9 does not insert new genes. It's a nucleoprotein complex which simply creates a double-stranded break that allows for the opportunity for DNA to be inserted during the repair pathways. For a gene to be inserted, it must be first localized to the break site, then the right pathway must be initiated to insert the gene, and finally the whole sequence must be inserted to be functional.

In therapies where the patient's own cells are removed, edited, grown, and then transplanted back into the patient, the unlikelihood of a gene to be inserted correctly doesn't matter as much. With millions of cells, we only need a percentage to take up the gene, and then we only need to screen for those lucky few to culture for the transplantation. This is also complimented with cell culture techniques where we can arrest or cycle the cells in specific modes where it favors the better repair pathway. Delivery is also easier as we can use electroporation to insert (relatively) large payloads of genes or machinery. The same applies for edits within other cells; if we can culture them then it is trivial to "insert" a gene.

This is drastically more complex for editing in vivo, where we want a pre-existing population of cells to take up those inserts. For this challenge, we need other tools, such as CRISPR-Prime, PASTE, or different "flavors" of CRISPR-Cas proteins; from nature or designed ourselves. This is still a work in progress, and even delivering a gene of significant size is a challenge.

Generally speaking, the mechanics of the insert are extremely specific to the domain and objective of the research or therapy. If you are wondering about a specific development I'm happy to look at the paper to parse it for you.

Source: Former CRISPR-Cas3 researcher