- when hiring, some companies prefer study cases while others like aesthetically pleasing images on the portfolio

True, those who focus on aesthetics need visual designers though, not UX/Product designers, but they may not know that.

- do hiring managers have the actual time to sit and read all study cases from the submitted portfolio and understand their purposes?

Only before the preparation for the job interview. Until then, they only scan through the case studies because it's so easy for an experienced manager to tell apart the good ones from the bad ones.

-are hiring managers unbiased when looking at candidates' portfolio, and how many can honestly say "Oh snap this guy is better than me, better discard his application"?

This doesn't happen in mature companies, but yeah it can happen in immature ones. I've worked for a Tech unicorn in sfotware developlemnt and their attitude in hiring was always to hire those who are better than us and bring new skills/experience to the table. That's the right attitude but the opposite can be common too. Sadly.

- can the hiring staff understand what's inside a designer resume and portfolio, can they tell the difference between Product Design, UX Design, UI Design?

Not sure which hiring staff you mean here. Recruiters generally have enough experience to tell these apart.