Usually the NATO countries have temporary and sometimes permanent deployments in other countries. So it is to expect that in the near future, there will be e.g. US, French, Italian troops stationed in Finland or Sweden for some combined exercise over a few months and in rotation with other NATO countries. After the Russian invasion in Ukraine, the US moved troops of the 101st airborne to Romania, close to the border to Ukraine.

Which is the super cool living proof of NATO article 5. If one would the start an attack on Romania or in the example on Finland or Sweden, he would also attack other countries’ forces at the same time that are stationed there. So it’s not just a combined defence because of some letters on a paper. The troops are already there and would get attacked as well.