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When your barista experience expands to include different beans and roast profiles, you might be surprised at the flavours you extract. Perhaps they're good, perhaps not so.
Here is a broad principle to help you navigate this grand adventure.
At the risk of over-simplifying, I'd suggest there are two sets of flavours in an espresso shot. The first set are the smoother, sweeter flavours that are more likely to be characteristic of the specific coffee. The second set are the more generic, intense and toasty flavours that come from roasting a seed to 200 degrees or so.
We roasters don't always like to admit that both flavour sets are enjoyed by coffee drinkers. Here's a graph to help explain...
Sweet smooth characteristics tend to come through early in an espresso shot. It seems most helpful to consider that their strength wanes over time. Meanwhile, the dark roasted notes seem to build intensity more slowly before they peak. Traditionally, a shot time of 25-30 seconds would achieve the desired balance of the two flavour types.
This is the sweet spot.
When a shot is too short, the sweet smooth notes may not achieve a strong enough coffee overall. Shots running longer than those in the zone, may no longer present their individual characteristics and be described as 'too strong', 'burnt', 'over-extracted', 'bitter' and so forth.
Of course, this is affected by the dose. My best advice is that adjusting the dose moves the sweet spot from lower left to upper right. It's like it moves the sweet smooth line left (small dose) or right (large dose).
A 1990s 15 gram espresso will be presenting mostly dark roasted notes within 30 seconds. Meanwhile, a modern espresso basket of 21 grams may still be delivering characteristic notes out past 40 seconds.
This is how I've come to understand espresso extraction. I'd love to you hear how it sounds to you.