In a rural slice of California’s Solano County, between the cities of the Bay Area and Sacramento, rumors have been swirling for years about “the Flanneries,” a mystery company buying up mostly undeveloped land.

At a shooting range in Birds Landing, an unincorporated community accessible by a two-lane highway or a gravel road through grassy foothills covered in wind turbines — many of them over 200 feet tall — an employee questioned why anyone would want to buy land in the quiet area.

“There’s sheep farms, there’s cattle ranches, and guys that are doing hay and safflower farming,” said the employee, Ashley Morrill, 40. “That’s what they do. There’s livestock, and things to feed the livestock.”

Solano County’s rural roots are still front and center in an area where a company backed by tech industry billionaires has been buying up land to create what they imagine to be a city of the future. That company, Flannery Associates, has committed roughly $900 million to secure thousands of acres of farmland, court documents show.

The cities of Vallejo, Fairfield and Vacaville, which are home to the majority of Solano County’s 450,000 people, aren’t very far away. But this part of the county, which covers about 900 square miles in all, has more in common with the farms of California’s Central Valley than the corporate campuses of Silicon Valley. And the prospect of big changes has unnerved some families that have lived in the area for generations.

Down the two-lane road a few miles from the range is Collinsville, an unincorporated community that’s essentially a mile-long, dead-end street with about a dozen houses, farms and silos along it. It backs into a marsh near the mouth of the Sacramento River. Property owners in the neighborhood said the mysterious Flanneries had approached them, and a few who have left abruptly apparently sold their land.

On a hot Sunday afternoon, as the air began to smell swampy, Lacey Miles was helping her retired father, Tom, unload his car in the driveway of his single-family home. Across the street was a recreational vehicle with a yellowing sign that read “For Sale” amid five-foot-tall hay grass.

Mr. Miles, 71, said he was concerned that the buyers were trying to change the countryside that he had lived in and enjoyed for decades. The only sound behind him was the low hum of wind turbines turning a few miles away.

“That’s why we’re here, the quiet community,” he said. “Love it out here.”

Ms. Miles, 42, who owns a housekeeping business, lives a few miles away. She had heard about the plans to build a “private city” on Facebook, and was opposed to the changes it would bring.

“I moved out here to escape the city,” she said. She had grown up near Collinsville, then moved away and came back 14 years ago with her husband to raise children in the rural area.

Ms. Miles said the people who hadn’t sold their land were likely to be opposed to any political push to create a new town. But she said with a sigh, “Anything is possible when you have money.”

In nearby Rio Vista, a town of about 10,000 people, most residents who spoke to The New York Times were aware that a coalition of Silicon Valley investors had been buying up farmland outside town.

The mystery buyers had been a subject of discussion in the town for the past few years, with theories ranging from more development for the wind turbines that dot the surrounding hills to an attempt to build another Silicon Valley to some foreign interests doing who knows what.

Downtown Rio Vista was right around the corner from a tractor shop, a recreational vehicle repair shop and a walkway along the river that men fish from starting in the early morning. It was a stretch of a few blocks lined with American flags and a street art project with differently painted ceramic sheep.

Pickup trucks and sedans were parked in the spaces along the road. A few drove down the street playing country music with the windows down. Older people wearing cowboy hats gathered in Raul’s Striper Cafe, which is filled with 1950s memorabilia.

More residents gathered at Foster’s Bighorn, a watering hole displaying hundreds of mounted animal heads on the wall, including a moose, a buffalo, a giraffe, a lion and a snow leopard.

Some residents said they were relieved to know the identities of the land buyers. Others were still concerned, and didn’t want the area to be flooded with techies. A bartender at Foster’s Bighorn said that whatever this new type of city was, it would price current residents out — a lot like all those Bay Area cities to the south.