how do i post announcements in different sections at the same time.
Announcements are designed to be sent to the students in a course one Canvas course at a time. If you were to cross list the sections in to a single Canvas course, then you could send a single announcement out in that course and all students would be able to see it. If you are interested in cross listing, please see How do I cross-list a section in a course as an instructor?
would you know how to link the courses to one another
The guide Robbie provided in his response gives details on how to link (cross-list) courses.
You may also be interested in adding your use case and vote to Post announcements in multiple courses at once
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