There is only one "technique" you need to master... STOP DISCIPLINE! As to "what trades to take?"....look for "behavioral correlations via technical analysis". There are LOTS of correlations... some more highly correlated than others. Study the charts and remember the famous words of Yogi Berra... "You can observe a lot just by watching."
only a idiot would not recognize there are people who have published highly successful methods which have made many people money who used them. i find the bitter people who scoff at the likelihood of a successful publisher usually enjoy crying victim.
Markets/Sectors, it doesn't matter, the point was being made, sometimes one market is booming while another is stagnating, hence ultimately try and cover several in order to offer opportunities at differing times. Personally for me trading forex, stocks, options, futures, crypto or other different markets (eg overseas) is too complex. Sectors is probably better (or easier) as they too can run at different tempos without the added complexities.
I day trade one concept...the trend pullback. As "trends" can be in conflict within my three time frames (intraday), I prefer the term momentum impulse, as there are variations of these based on which tf they setup and have respective variations of filters and qualifiers. I get plenty of signals each and every week, typically 12-15 on a very slow week and 60 or more on an active week. Bottom line...conceptually simple but the trade plan itself is rather complex and was developed and refined over many months. "Simple" is a relative term with respect to trading and if you're going to succeed, you have plenty of work to do.