This is just an "elevator speech" definition, not the whole body of knowledge. Systems Engineering is an engineering major; it takes several years to get a degree, and usually several years before that to get an engineering degree in a specialty area. If you are interested in the required qualifications, there is a lot of material for that as well; e.g., the EIA-731 "Systems Engineering Capability Model" standard. Or see e.g. https://www.sebokwiki.org/wiki/Guide_to_the_Systems_Engineer... which has at least a summary of nearly every relevant aspect of the field.