How is a part written?
A whole bitcoin can be divided into 100,000,000 satoshis, often called sats. An amount can be written in BTC with decimal places or as a number of sats. You are still describing bitcoin, not a different coin.
Being able to describe a small amount does not mean every service will handle it. Fees, minimums and withdrawal terms can affect the amount a service allows.
What does a fraction look like?
For an amount example, 0.01 BTC is 1,000,000 sats. This is a conversion between units, not an example of what it costs in dollars. A price quote would depend on the service and the time.
If an account offers fractional amounts, you still need to ask who holds the keys. A balance in an account does not automatically mean you can withdraw that amount or control its spending keys.
What should you read first?
Read minimum purchase and withdrawal amounts, the stated fee and the spread between buying and selling quotes. Learn what happens if the service stops operating. Understanding a fraction does not require making a purchase.