The ships move along scheduled routes which are not their real life routes but realistic routes for each ship model and repaint. There are a lot of ferries included and they are on their real routes but not on their real schedules. You will find little traffic in the middle of the Atlantic as it wouldn’t serve much for must simmers, but in general you will find a traffic which is regionalized across the entire globe, so if you fly in Brazil you will see typical ships from that country e.g. naval vessels and fishing boats or large ore carriers - if you fly off the coast of China you will see Chinese Naval vessels and fishing boats but the same large ore carriers at the other end of their trip…
You will also see that Navy vessels are operating together - e.g. there is a large landing exercise off the Danish military terrain Oksbøl with ships from around 10 different NATO countries - or in the screenshot above a line of Royal Navy ships returning to the their home port.