Just type "what is my user agent" into Google, it'll tell you as well.

Do sites think that I'm visiting in with Linux Mobile or do they think it's Android OS?

Most will think you're running Android because they're either lazy, incompetent, or ignorant.

The user agent contains something like "Linux Ubuntu like Android". Devs just do a quick check, see that it contains "Android" and assume that's what's running.