EA: what is core? trading systems? trading criteria? etc..

Hit at all,

I'm studying a simple EA taken from the example: Simple Expert Advisor https://book.mql4.com/samples/expert.
I wrote the code from scratch (for a better understanding), following the same procedure of the framework,
the same trading systems, same trading criteria, values parameter etc. etc.
I'm still continuing to study and improve the code,
I hoped I had reached a point enough to put it in a bit of backTesting and see results,
but I get bad results

What do you suggest me to better it?

what is core of a EA profitable ???

trading systems / trading strument ?
trading criteria ?
values parameters ?
framework ?

Thanks a lot to everyone in advance, of your time and your skills

sometimes a good EA will use just ONE indicator, but a very good one, sometimes you will conbine two indicators or more

there is no absolute rule