How are shared boundary ways organised?

Hi, I am doing processing of relations, ways and nodes for administration boundaries to work out which relations border each other. My algorithm uses an assumption and I’d welcome some feedback on whether this assumption is correct.

Say there is a boundary admin level 2 running east-west. Country A is to the north and country B is to the south. The relations will share a way.

If I now zoom in and look at a relation in country A of admin level 12 that also borders country B is it true that:

  • There is a shared way between the country A admin level 12 relation and the country B admin level 2 relation
  • The extent of the shared way is just the extent of the shared border
  • The border between country A and country B is actually divided into lots of short ways to accomodate all the relations of different admin levels on each side of the border

Or is it that the ways that make up the country A admin level 12 relation can join with the country level shared way at nodes anywhere along its length?

I am really hoping it is the former. Hope this question makes sense.
Best, Chessel

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