The wolves howl to say : "whoever hears this, you are on our territory".

Dogs and wolves respect boundaries, and can be stressed simply at the idea of being on the territory that another dog/wolf claims.

Howls don’t have a meaning as in "a short yap followed by a long tremolo means hi". Maybe howls can have significances like wolves may howl a bit differently depending on their mood, the moon, a specific event or whatever. They may share this feeling through their howls, anyone (dogs, humans, animals,…) could maybe interpret specific howls and attribute vague meanings but…

No, dogs stress out because howling is claiming territory.