Is OP talking about arbitrage trading where you use price differences across different exchanges to your advantage? This is difficult to do with BTC as most exchanges are pretty good with up to date bitcoin prices these days and tx fees and times make it impractical. and you would need significant capital to make any profit.
people do this in nft marketplaces like opensea. if you closely watch the live trades you can clearly work out the successful bots that are working to match profitable trades. i tried to make one but it is difficult to compete with the existing bots on the site.
trade-fi (old language) talk about different exchanges. but crypto can do it via going through same exchange market tradepairs on the same exchange because crypto allows different trading pairs to internally do swaps
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but even running through the trading pairs to arbitrage cycle back to base asset on the same exchange. the timing of doing it manually vs bot trading means opportunities do not always occur or last long, so difficult/risky to attempt unless lucky
the main risk is. you cant be a 'maker' where you set a preferential price and add it to market orderlist for someone to take. and do it again on next trading pair and next to get back to base to get profit at set order prices..
instead you have to be the taker.
taking whatever orders others have set so you can rapidly arbitrage the trading pairs back to base asset.. if every other bot is doing the same you might be stonewalled mid way and end up taking a order at a loss compared to the situation when it first arose half a second earlier.
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when they say "make 20%" a year
what they are talking about is not a one shot wonder of 20% win.
its instead not using a lump sum amount but small allotments to not hit walls.
lets say you have 0.1, but most orders are 0.001
so you can only play 0.001 at a time
each time you play and succeed. there is 3x 0.1% fee meaning 0.3% fee deducted.
and so you need to see opportunity of atleast 0.4% (super risky) spread over the arbitrage cycle to cover fee's just to earn 0.1% profit of the 0.001
so each play ends up being 0.00101 (0.00001profit)
so to turn your stash 0.1 into 0.12 (20%profit) you need to do 2000 plays a year (5-6 plays a day)
spreads per tradepair used to be wide years ago. arbitraging was super fun in the 2012-14 days. but since bots took over on most exchanges the opportunities and dried up and most times end up getting stuck mid arbitrage faced with having to take an order at a price you didnt want to take