How do they work? Do you have to go into each exchange and download transaction history, or can an aggregator access various exchanges (given user id info, etc.):
Koinly for example works by providing it with your API key from Binance or Coinbase or other exchanges, at one point least some of them had also the possibility of a full dump of your transactions, but never ever entrust some random app downloaded from Gplay with your username and password.
This year it is a different matter though. I did something really stupid and accidentally sold a bunch of crypto.
If it was a single event even with multiple cryptos involved I don't think you need one, but if the purchases are spread and you also used different exchanges, yeah just go for an aggregator. There are a bunch that offers a free plan for under 100-1000 transactions.