As far as I know, Apple Watch does not need a passcode or something to pay with Apple Pay nor unlock Apple Watch (but probably it can be set).
Now let's talk about Apple Watch 6 Cellular version in situation when iPhone is not nearby (I left it in home).
As we know, when we want to pay with Apple Pay on iPhone, the phone requires TouchID or FaceID to identify that "we" are we.
Does a similar identification mechanism exist on the Apple Watch? Does Apple Watch know if really "I" am wearing the Watch or when my watch would be stolen, Apple Pay can be used by anybody else as long as then will use it when it is on someone's wrist?