Except for the second couple of sentences, all the other sentences have something in common: They negate a verb.
If you are negating the verb in a question:
- If the verb is be, you replace are with aren't, and is with isn't
- If the verb is another one, you generally replace do/does (the auxiliary verb) with don't/doesn't
If you are negating the verb in a sentence that isn't a question:
- If the verb is be, you replace are with aren't, and is with isn't
- If the verb is another one, you generally add do/does (does is for the third singular person) before the verb
In your examples, practice, come, marry, care, realize are all verbs, and they are not be.
"What is it?" is correct if for example you are asking what the object you are looking is. "What does it?" would eventually be "What does it do?" and it has a different meaning.
Notice that matter is also a noun; as such, it used in sentences like the following ones:
It is a matter for the police.
It is not a matter of not being able to do something; it is just a matter of knowing the right people.