What is the minimum amount of Ethereum you can buy?

Could you please elaborate on the question, "What is the minimum amount of Ethereum you can buy?" I'm particularly interested in understanding the technical and practical constraints surrounding this topic. Does the minimum vary depending on the exchange or platform? Are there any associated fees or charges for purchasing smaller amounts? Also, how does the minimum amount compare to other cryptocurrencies? Is there a standard minimum across the industry, or does it vary widely? Thank you for addressing these points in your response.