Urban planning and public transport should absolutely go hand-in-hand.
But on to your other point.
The key factor for transport use isn't just population or density. It's also the proportion of the population that uses public transport. And places that have more frequent public transport will have a higher proportion of the population using it than places with low quality public transport.
Imagine a city with just 100,000 people. But the local bus service is exceptional, and half the population uses it. That's a base of 50,000 people.
Imagine a city of 500,000 people. The public transport network there is average, so just 10% of the population uses it. That's 50,000 people.
Now imagine a metropolitan area of 5,000,000 people. The public transport network there is poor and infrequent. Only 1% of the population uses it. That's 50,000 people.
Three cities, same absolute number of public transport users, different modal share.
If you run frequent services, every 10 minutes or better, and services connect so that it's a two- or three-seat journey to everywhere in your city, you will have a much higher ridership than if it's an hourly bus service. That's with the same population and density.
Frequent bus services (once every 10 minutes or more) can also act as a feeder into a higher rail, light rail, tram, or metro services. In suburban, rural, and seni-rural areas, that extends the reach of your rail network.
Yes, higher density around railway stations is the best option. But where there is a lot of low-density suburban sprawl, frequent feeder buses are a good option.
@Adori@ray Even in many rural areas, this is not the best option.
First, in many towns, there often aren't any Uber drivers nearby, or the nearest driver is in another town and you're left to wait upwards of an hour for your ride to arrive.
Second, pairs of major cities and large metropolitan areas that are relatively close together should be connected by a railway line. Along with express services, these railways should have reasonably frequent all-stations services that serve the smaller towns along the way.
Third, there should be regular bus or coach services connecting multiple towns, and where available, feeding into these all-stations train services.
So if there's a train station in town A, there should be a feeder bus to nearby towns B, C, and D. This benefits rail passengers, who have more towns they can visit by public transport, and connects those towns to the rail network.
These inter-town bus services can make multiple stops in each town (for example at the local school, the local shops, and the local hospital),, providing both cross-town and inter-town services.
Fourth, with public transport, one service or route won't cover every pair of destinations—but a network can.
So say you have an east-west bus route connecting towns A, B, C, and D. You might have a second route that connects with that bus service at town C, and then runs north-south to connect it with towns E, F, G, and H.
The number of people travelling from town H to town D might be vanishingly small—zero on most days, no more than one or two on others. Certainly not enough to run a dedicated service from town D to town H.
Yet that trip can be provided for by the network, which draws its ridership from passengers who want to travel from any stop on either the north-south or east-west service, to any other stop on either service.
Fourth, with larger towns over 1000 people, an on-demand bus service that travels around town to designated stops is probably a better option. Again, this should feed into any railway stations of inter-town bus routes.
And finally, once your city reaches a population of around 10,000 or so, it should just have a regular bus service, and it should integrate with the broader bus and train network.