For the 189 you don’t need a job before applying for a visa , just a positive skills assessment, for state sponsorship you will need to check what the requirements are for each state re having a job , some may require it , also for 190 some states like Victoria require at least registration in principal with AHPRA before you apply for 190
most employers now will require your visa to be granted before you are considered for employment, there isn’t a shortage of nurses in Australia, and most employers will only consider applicants who have the right to work in Australia already