Under "on Tick" each block that is on top level (nothing connected in its white input) runs on every tick. When you have group of blocks and one of them can't pass, then this is the last block to work. But if you have multiple paths after one block, each path will run. Look at these examples https://fxdreema.com/examples/#Interface
MetaTrader already provides arrays with candles data and indicators data. In all moments you can get O, H, L and C of candle 17 for example. The same goes for incdicators - by prividing Candle ID, you can get the value from that candle just like that.
It's a little bit harder when you don't know the ID of the candle from which you want to get a value. But it depends, some things are already avalable, like the Highest value of N candles for example. Also, for custom indicators (My indicators) there are extra options in More settings