FREMONT, Calif. — It was just past sunrise on an early-spring morning at Netflix’s DVD operations here, where metallic arms whirred in a giant glass box and rolling carts holding millions of DVDs lined the walls. The company’s iconic red envelopes buzzed through an assembly line at the other end of the warehouse.

The machine sucked a returned Netflix mailer into the system then proceeded to slice open the envelope, identify and clean the disc inside, check that the DVD worked and reinsert it into the original sleeve. That disc was then returned to the storage carts or shipped out to another customer who had requested the title.

About 3,400 discs zip through the rental return machine each hour, five times as many as when teams of Netflix employees used to process the discs by hand. Called the Amazing Arm by engineers here, the machine symbolizes the way Netflix has managed to maintain a profitable physical DVD operation even as it transforms itself into a global streaming service.

Netflix now counts more than 65 million streaming members in more than 50 countries and plans to expand across the world in the next 18 months. But that breakneck growth comes at a cost: The company expects its streaming business to just break even globally through 2016 as it pours billions of dollars into content and an aggressive expansion.

Helping fuel that expansion is the company’s dwindling, often ignored DVD-by-mail operation, known for envelopes that wind up under sofa cushions and viewed by many as an anachronism in an era of lightning-fast streaming.

Netflix has 5.3 million DVD subscribers, a significant falloff from its peak of about 20 million in 2010; still, the division continues to churn out hundreds of millions of dollars in profit each year. And behind the scenes, engineers are trying to improve customer service and streamline the labor-intensive process of returning, sorting and shipping millions of DVDs each week.

Netflix has not put a life expectancy on its DVD division. Even as its subscriber count shrinks, the group has kept a core base of customers, particularly in rural zones with lackluster Internet service and among people who want access to the breadth of its selection, and executives expect it to stay around. To hold on to those customers — and the profits they bring — Netflix continues to deploy innovative technologies that help trim costs as well as improve customer service.

“If you cut back on service, you are going to lose your subscriber base,” said Hank Breeggemann, general manager of Netflix’s DVD division, who has worked for the company for 13 years. “Expect us to continue to ship DVDs for the foreseeable future.”

Here at the Fremont hub, Netflix used to employ about 100 people to handle the returning, sorting and shipping of the DVDs. Today, about 25 employees work through the night, largely assisting the machines. Their shifts start at about 2 a.m.

By 8 a.m., the discs are out the door and the steady buzz of the machines starts to fade.

“Embrace change — that’s what I’ve learned here at Netflix,” Breeggemann said. “If you don’t like change, this is the wrong place. Something is going to change every single day.”

If anything, the Netflix story is one of perpetual change. The software executives Reed Hastings and Marc Randolph founded Netflix in 1997 to offer online movie rentals by mail. Netflix introduced streaming in 2007 and plans to be fully global by 2017.

Netflix’s transformation has been rocky at times, but its evolution has become an example of how companies can adapt, tapping their legacy businesses to fuel growth in new areas as the ground underneath them shifts.

Four years ago, Hastings, Netflix’s chief executive, said one of his greatest fears was that the company would not make the leap from DVDs to streaming.

At the time, Netflix was suffering a humiliating blow after its disastrous attempt to raise prices and split into two companies — one for its DVD mailing group and another for streaming. About a million customers canceled their subscriptions, while

Netflix’s share price plummeted to less than $53 from $300.

“Most companies that are great at something — like AOL dialup or Borders bookstores — do not become great at new things people want (streaming for us) because they are afraid to hurt their initial business,” Hastings said in a September 2011 blog post, in the midst of the tumultuous period.

“Eventually these companies realize their error of not focusing enough on the new thing, and then the company fights desperately and hopelessly to recover,” he continued. “Companies rarely die from moving too fast, and they frequently die from moving too slowly.”

Within months, Netflix had publicly abandoned the separation. But behind the scenes, it quietly divided the company in two. The streaming side focused on luring subscribers around the globe and producing original series like the political drama “House of Cards,” while the DVD division focused on managing the subscriber decline of the mail service by making it more efficient. The two groups have separate management teams, headquarters that are about 25 miles apart and different employee incentives.

“What’s interesting is that although the business is in a slow decline, there is still a huge demand there,” Breeggemann said of the DVD side, noting that Netflix had about 93,000 titles on DVD and next-day delivery service for 92 percent of its subscribers.

Recently released films tend to be available only on DVD and not on the streaming service because of rights issues.

At its peak, Netflix operated about 50 distribution centers across the country. Now that number is down to 33. The introduction of automation technologies has allowed the company to process more DVDs and expand service areas. Netflix also has reworked its schedule to sync with new delivery standards at the U.S. Postal Service.

“Yes, we still do DVDs,” Breeggemann said with a laugh, responding to a comment that many people were not aware that Netflix still provided discs. “It is a completely different company.”