The MetroPlex is now just the name for the Dallas/Fort Worth statistical metropolitan area, but it was very much planned in advance and given that name in the 1960s. Once upon a time, Fort Worth was a relatively small cowtown and Dallas was a separate city a long 30 miles away with an entirely different culture. In between were a number of very small towns, all of which have grown to capacity. Part of the plan to create the MetroPlex involved Fort Worth absorbing a mile-or-so strip of land eastward until it reached the old American Airlines headquarters airport and Dallas doing the same westward to reach the same point, and then build what is now known as the DFW airport, really the first modern airport in the country. Two thirds of DFW is in Tarrant County (Fort Worth is the county seat), mostly in Grapevine, and the other third is in Dallas County, mostly in Irving. The original discussions about creating the MetroPlex included plans for Dallas and Fort Worth to incorporate each other and become one city with that name, but in the end the cultures were two disparate, and that idea entirely disappeared.
The airport was being built as I was in high school in Grapevine, and it has transformed Grapevine from a 6000-resident ranch town that operated almost entirely separately from Fort Worth and Dallas into basically a suburb of Dallas. Arlington, which is closing in on half a million residents, had a population of 5K in 1950, 50K in 1960, and almost 100K when I first left Texas in 1972. I lived in Colleyville back then, which didn't even have its own junior high or high school, and our back yard backed up to a large horse stud farm. Colleyville still retains some of its flavor from 50 years ago, but for the most part every bit of the MetroPlex (all but downtown Dallas), which encompasses 6 counties now, is unrecognizable compared to the 1970s. Tarrant County is currently the fastest-growing county in the United States (Dallas County is already long fully-developed).
Government is always spinning facts to fit certain narratives, and how this in some ways affects the MetroPlex is an example. Shortly before Trump left office, the MetroPlex edged past Chicago the Census Bureau statistics to become the third largest metropolitan area in the country, but the whole Wuhan Flu mess had emphasized to the country that certain areas were thriving while others were tanking, with Texas accounting for over half the economic growth of the entire country (five of the fifteen largest cities in the country are in Texas), so one of the first directives the Census Bureau got from the Biden administration was to beef up numerous other metropolitan areas, creating a new designation called Primary Statistical Areas, which then raised Chicago, DC, San Francisco and Boston ahead of the MetroPlex. In order to accomplish that, they had to pretend that everything in Connecticut, Rhode Island, Massachusetts and southern New Hampshire is part of Boston; that Silicon Valley is part of San Francisco (of the four actually the least of a stretch), that everything from Delaware and all of Maryland to northern Virginia and eastern West Virginia is part of the District of Columbia; and that Chicago extends not only into Indiana but Wisconsin as well (think Kenosha). Money allocations get based on these kinds of decisions, but some of it is just absurd. Personally, I liked the Fort Worth area much better 50 years ago, but now it's unmistakably one nearly continuous city from western Fort Worth to eastern Dallas. At the same time, no one is going to mistake the decreasing-population New England environment from Boston to Connecticut for anything that resembles one long city.
Wow -- this is the definition of over-answering.