It can vary wildly country to country. Some have a small quota of sick days that don’t require doctor’s note, some require it for every day. Most started allowing getting doctors note remotely during the pandemic. Governments here generally take more interest in it because social security is usually expected to pay at least some part of your salary during sick leave. There’s also a separate concept of „leave on demand” / force majeure leave that can be treated similar to those sick days without doctor’s note (EU started to require it recently).