Supernoob buying bitcoin question

This is the most basic obvious question about buying bitcoin, idk if I just dont have the right keywords to search or what. Wondering why when I buy 100 dollars worth of bitcoin, it ends up being like 85 dollars worth in the wallet. Then when I buy something, there are more huge transfer fees. I'm sure im using a shitty medium (bitcoin.com wallet) but do they just have huge fees or is that just kinda how bitcoin works? I just buy the bitcoin every once in a while when I want to make a single purchase, and yes i know I should prob just buy a decent amount and keep it in a wallet but I don't, so by the time I buy bitcoin and want to buy something for say 70 dollars worth of btc it ends up costing me like 25 dollars extra. I dont want to get too deep in to the tech side of it, rather just transfer money from a card in to bitcoin and buy something but is it just always expensive doing it that way? TIA