What is a purchase fee?

Excuse me, could you please elaborate on what exactly a purchase fee entails? Is it a one-time charge that I have to pay whenever I make a transaction, or is it a recurring fee? Additionally, does the purchase fee vary depending on the cryptocurrency I'm buying, the platform I'm using, or the amount of the transaction? It would be greatly appreciated if you could provide me with a clear and concise explanation of how purchase fees work and what factors influence their amount.