There are essentially 4 rules for effective advertising:
1) ignore all metrics from online platforms (especially the ones that sell ads)
2) ignore all metrics from ad agencies
3) run tests that make sense*
4) measure in dollars
The hard part is convincing the founder/owner to stop messing with stuff long enough that you can actually measure the results. Also, companies tend to way overspend on ads and data scientists while underspending on skilled advertisers. (They can be forgiven for the latter, there are too few skilled advertisers, and lousy advertisers usually sell themselves as experts.)
*Example: your job is to advertise a business with 20 locations in 2 cities. You want to know if radio ads are better than search ads. So you do the obvious thing: buy radio ads in city A and search ads in city B, then compare sales. If the difference in sales is big enough that managers get defensive and start pointing out why the test wasn't valid, you may have a winner. Run it in both cities and see if it still works.
(Rule of thumb: if nobody is upset with the outcome, the variations were too similar. Run bigger tests.)