Just curious what happens if your thai spouse passes on or you divorce and you are on a marrige visa ? How do you stay in country ?

You don't, at least not based on marriage.

When the reason of the extension no longer exists, your permission to stay ends. In case of a divorce, it means you have to leave the country the same day. In case of death immigration does not seem to be that strict and will allow you time and don't expect you to leave the country straight away. However, it does mean your permission to stay ends.

You would need to qualify for an extension of stay for another reason, like retirement or having a Thai child to continue to stay in Thailand.