Learn Finnish: Miten se tapahtuu?

Yes! While milloin asks "when" and miksi asks "why", miten is the question word for "how". Just like the others, it sits at the very beginning of the sentence to form a question, without needing any extra helper words like "does" in English.

Where is the word for "does" in the Finnish sentence?

Finnish doesn't use helper verbs like "do" or "does" to make questions! Instead of saying "How does it happen?", you just put the question word first and use the normal verb: Miten (How) se (it) tapahtuu (happens).

Why does tapahtuu end with a double 'u'?

This is the standard present tense rule for "he/she/it" in Finnish. For the third-person singular (with hän or se), you take the verb stem (tapahtu-) and double its last vowel. That makes it tapahtuu (it happens).

What is the dictionary form (infinitive) of tapahtuu?

The dictionary form is tapahtua (to happen). When you conjugate it for se, you drop the final -a to get the stem tapahtu-, and then apply the vowel-doubling rule to make it tapahtuu.