So let's say I've found two different exchanges where prices of selected cryptocurrencies is varying quite a bit.
How to arbitrage?
Should I buy some BTC at let's say 3.700 then send this amount to second exchange where I will sell it for 3.900? The profit here is that I have more fiat and I can't really multiply this process (or maybe I can send earned money to the first exchange again, but it takes a lot of time).
Am I seeing this right?
In theory yes you could do this. In practice, it takes so long for bitcoin to confirm that the price might have moved in the meantime. In addition, fiat also takes a while to move (about 3 days and they don't move money over weekends).
However, you should be able to do arbitrage with a coin that confirms in a few minutes.