Not sure what you mean by "sourced through the employer".
Usually you need to have enough qualification and a job offer to apply for a work visa that is bound to the employing company for two years. Switching jobs requires reapplying. Afterwards, you can usually get a work visa that is not bound to a specific company, and less time limited.
Normally it's you applying for the visa, but in some cases the company hiring you can file the application, which can drastically reduce the time you have to wait, but cedes some control.
Of course, all this depends on your nationality and the country where you are applying.
Usually you need to have enough qualification and a job offer to apply for a work visa that is bound to the employing company for two years. Switching jobs requires reapplying. Afterwards, you can usually get a work visa that is not bound to a specific company, and less time limited.
Normally it's you applying for the visa, but in some cases the company hiring you can file the application, which can drastically reduce the time you have to wait, but cedes some control.
Of course, all this depends on your nationality and the country where you are applying.