Learn Russian: Это миллион долларов?

Because it comes after миллион. Words expressing a quantity demand the genitive case to say 'of something' (literally 'a million of dollars'). Since доллар is a masculine noun ending in a hard consonant, its genitive plural is formed by adding -ов.

Is миллион a number or a regular noun?

Grammatically, it acts as a regular masculine noun. Just like you would say 'a box of dollars', in Russian you say 'a million of dollars'. That is why it forces the next word into the genitive case.

If миллион is masculine, why does the sentence start with the neuter word Это?

When you use 'This is...' or 'Is this...' to identify or point to something, Russian always uses the fixed, unchanging word это. It does not change to match the gender of the noun that follows.

Why is there no word for 'a' or 'one' before миллион?

Russian does not have articles like 'a' or 'the'. The noun миллион on its own translates perfectly to 'a million' or 'one million' in English.