The iPhone 12 lineup has changed Apple's (NASDAQ:AAPL) fortunes in 2020 by quickly becoming the top-selling 5G smartphone in the world, saving the tech giant from a potential sales slump in the wake of the novel coronavirus pandemic.

Counterpoint Research reports that the iPhone 12 and the iPhone 12 Pro together grabbed 24% of the 5G smartphone market worldwide in October, which is remarkable considering the devices were on sale for just two weeks (including a week of pre-orders). But the iPhone 12's terrific sales response was already in the making thanks to Apple's pricing of the new devices, the addition of 5G wireless technology, and a huge installed base of users waiting to upgrade.

Apple stock has also benefited from the iPhone 12-related developments, shooting higher in the past three months to end the year on a solid note.

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But don't be surprised to see Apple keep up its impressive momentum in 2021, as the iPhone 12 could push the company's smartphone sales to new highs -- and this is just one of the many catalysts to watch out for in the new year. Let's see why Apple could remain a top growth stock in 2021.

The iPhone 12 would be a big tailwind for Apple

The iPhone was Apple's biggest source of revenue in the recently concluded fiscal 2020 (ending on Sept. 26, 2020), producing just over half of the company's total revenue. However, the iPhone category's revenue was down around 3.2% over the prior year. That wasn't surprising, since the COVID-19 crisis dented the smartphone industry in the first half of 2020.

But the launch of the iPhone 12 seems to have triggered a terrific turnaround for Apple in the final quarter of the calendar year. According to Wedbush Securities, Apple reportedly moved 195 million iPhones in 2020, an impressive performance that bettered estimated shipments of 185 million units last year.

Investors should note that since Apple has stopped reporting unit sales of iPhones and other devices, the numbers may not be entirely accurate, as they come from a third-party source. But Apple management predicts a year-over-year increase in iPhone revenue in the first quarter of fiscal 2021 (which corresponds with the final quarter of the calendar year), even though the devices hit the market later than usual.

The company didn't offer specific guidance, but higher iPhone revenue in the December quarter is an indication of strong demand for the device. The really good part is that the iPhone 12's sales momentum could continue well into 2021. A report suggests that Apple is looking to increase production by 30% in the first half of the new calendar year to meet surging demand for its 5G iPhone.

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For the full year, Apple is reportedly targeting production of 230 million units. That would be an increase of nearly 18% compared to 2020's estimated shipments. Also, it would be much higher than the estimated 5.4% shipment growth clocked in 2020. And Apple should be able to hit those numbers given the impressive market share that the new iPhones have logged already.

Chipmaker Micron Technology anticipates that sales of 5G smartphones could hit 500 million units in 2021. Apple already seems to be in a great position to corner a larger share of that market, and it may even exceed its targets if it continues holding on to its share. As such, Apple's biggest product looks set to step on the gas in 2021, giving investors a great reason to buy this tech giant.

However, this is not the only reason why you should consider going long on this stock.

More reasons to go long on Apple

While the iPhone product line is expected to be a catalyst for Apple in 2021, the company expects other areas to also contribute handsomely toward its growth. CFO Luca Maestri pointed out on the last earnings conference call that Apple expects "all other products in aggregate to grow double digits, and we also expect services to continue to grow double digits" in the December quarter.

Apple could keep up this momentum for the remainder of the year, especially considering the tailwinds in the services business, which supplied nearly 20% of its revenue last fiscal year. The services business recorded 16% year-over-year growth in fiscal 2020 and produced nearly 34% of the company's gross margin. Looking ahead, Apple could maintain its double-digit growth rate in the services business thanks to the launch of new services and a bundled subscription offering, giving the company's bottom line a nice shot in the arm.

All of this indicates that Apple is sitting on several growth drivers that could help it hit a higher gear in the new year. That's probably the reason why analyst estimates compiled by Yahoo! Finance point toward a 15% increase in the company's revenue this fiscal year, which would be a big jump over the 5.5% jump it recorded last year.

Investors on the hunt for a growth stock should keep Apple in their sights -- it not only stands to win big from the 5G space, but it also has other catalysts that could lead to better bottom-line performance and help it deliver more upside.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.