Is Yamaraj the one who decides what people will supposedly come back as?
yamaraj carries out what is elaborated upon by manu and which is established by visnu.
Like for instance, due to being envious of god, the living entity comes to the material world in the mood of sense gratification, as opposed to service to god.
While in the material world there are many scriptural recommendations on how to regulate one's sense gratification (eg - scripture gives warnings about eating food stuffs in the mode of ignorance and instead advocates foods in th emode of goodness or foods offerred in sacrifice)
One who fails to take heed of such instructions runs the risk of taking birth as something lower (if one is sincerely interested in eating anything and everything there are better life forms - like the pig, simialrly if one is interested exclusively in sex there is the pigeon, and why waste time at the gym when you can solve all your issues at birth by becoming an elephant). Thus they go to the hellish planets, under the decree of yamaraj to get "trained up" to become an animal.
One can also go th e hellish planets to get higher denominations of suffering than what the earthly planet (its a middle planet) can dish out - for instance if one is a mass murder it is more efficient to go to the hellish planets where one can get the results of one's previous actions more quickly - similarly if one is amazingly pious one can go to upper (material) planets for a better scopr of sense gratification.
In short, the laws of karma are incredibly difficult to negotiate, since there are questions of one's attachment to activity which can not be so easily legislated by simple "do's" and "do not's" - like for instance its not like everyone who has ever been to a gym will take birth as an elephant or that anyone who has ever had sex will take birth as a pigeon (that said, its no coincidence that the ratio of humans to animals is miniscule) ..... Therefore the conclusion of all scripture is that one should strive to serve god without material motivation, which is the surest way to achieve a good result, rather than striving to anticipate a great material reward for one's spiritual pursuits
Is there a Hindu god who created the universe?
Visnu
or more specifically from the Sātvata-tantra
"Viṣṇu has three forms called puruṣas. The first, Mahā-Viṣṇu, is the creator of the total material energy [mahat], the second is Garbhodaśāyī, who is situated within each universe, and the third is Kṣīrodaśāyī, who lives in the heart of every living being. He who knows these three becomes liberated from the clutches of māyā."
more detailed explanation
here, at half way down the page titled "A summary of the Vedic version of creation"
