A beginner shouldn't start making investments directly, but if they know they have understood things somehow and now want to start investing, they can start with as little as $10, or even less if someone is selling Bitcoin privately to you and you are not buying directly from the market. I'm not totally sure but I think P2P trading can allow lower amounts to be bought compared to open market where you place your order to buy using stable coins or other cryptocurrencies.

People who are often new to cryptocurrencies and have no knowledge about cryptocurrencies think and believe that you can't buy them in portions, and if someone wants to invest, they will have to buy one whole coin or token. That is the only reason why someone can think and say that Bitcoin is just for the rich because only the rich can afford to buy a whole Bitcoin now considering its price.
If someone comes to know that you can even buy fractions of it since it has 8 decimal points, they wouldn't say such things to begin with.