In gene psychoanalysis resembling for that of SCID where on earth do they modify the dna?

in gene treatment the people introduce a well-mannered copy of ADA producing gene in some cell and then inject them surrounded by the bone marrow of the pateints thus claim to change the genetic makeup of the pateint .But how do they tweaking the genetic makeup in adjectives the millions of the cells? In gene dream therapy, no one truly changes the productive DNA of the patient. Typically they simply provide a correct copy of the gene surrounded by question within the tissues/organs/cells that need the correct gene for the forgiving to be healthy again. Therefore it is not immensely important to hold the gene-therapy cells/DNA in every cell of the long-suffering; at least contained by case of SCID. But contained by some other cases like muscular dystrophy, the gene psychiatric help would need to be done within literally 70% of the body tissues.. now that's a existing challenge still waiting to be solved!