For the US there is USAJOBS.gov. Make sure you look for jobs that are 'Open to the public' No need wasting time looking at a job you cannot apply for. Since

For the US there is USAJOBS.gov. Make sure you look for jobs that are 'Open to the public' No need wasting time looking at a job you cannot apply for. Since Covid many of them have virtual job fairs. The clearance is the hardest part. You should be able to find discussions of that online. A major problem is the usual time it takes to get a clearance. Do not expect to have an interview and start in less than a few months. In the UK and US you have to be a citizen, not sure about other places. You do not need to be native. It is often easier to start off as a contractor and then get hired.