Persons who wish to have positions in multiple companies or who wish to work as a freelancer, without being employed by a single company, must also have a working visa (status of residence). Visas such as these are often called “self-sponsored visas,” but Japan does not have visas of this sort.
To work in Japan as a freelancer, a foreign national can apply for a standard working visa with a status of residence that corresponds to the work they will perform. The person must provide evidence that they have concluded stable contracts with various employers/customers in Japan and that they will earn a certain level of income.