Crypto fee help please

Hi all

I'm new to reddit and new to exchanging crypocurrency. Can anybody explain to me how the fees work on bit true please especially limit order fees. I seem to have been charged a load of different fees on every transaction I have made bar 2.

So originally I bought 650,000 coins in a crypto on advice from a friend ages ago. On this first transaction for some reason it doesn't show on my history. Perhaps because I did on the computer and now I'm all set up on phone. When I sold these coins the other day I was only charged 1usd for selling them all.

I then set a limit order to buy again at a lower price. I originally was only going to invest half of my sale money from previously. I then decided to use the whole of my sale money and set another buy limit order. For some reason I must not have pulled the tab across completely and it left me with usdt still in my account. I then set a third buy limit order for the remainder.

All these buy limit orders were for the same price. They went through when the price dropped but the fee for the 1st order which was the largest was only 2usd. The 2nd fee was 6 usd. The 3rd fee for the smallest amount of coins was 7usd.

I really don't understand this at all. Is this simply because I set 3 seperate transactions and it charges more qith each?

Should I have just cancelled the 1st one after changing my mind and set it as one trade?

Thanks in advance and yes I know I'm a noob I just want to know for the future.