For policies with less than 500,000 USD (or currency equivalent) of cover, from applicants under age 50 with no serious prior health conditions, no medical is required and your policy will be automatically underwritten at the cost quoted to you online. That will be the price for the entire duration of your cover.
If you require cover of more than 500,000 USD (or currency equivalent), have a prior medical condition, are over 50, or you need whole of life insurance (sometimes called "permanent insurance") for the purpose of tax planning or legacy planning, then your policy will be manually underwritten. The quoted rate may change based upon your health and you will be required to take a medical exam.
If your non-standard cover goes through the underwriting process and unforeseen risks are discovered (e.g health issues) you may be offered cover at an increased cost, if the insurer is willing, or refused cover altogether.