The map above shows the turnout and the candidate that won each county.  The only place where Beto O'Rourke won and the turnout was high was in Austin and its northern suburbs.  Every other county he won had either moderate or low turnout.  Ted Cruz, on the other hand, had really high turnout in San Antonio's northern suburbs and various other counties throughout the state.  He also won very few counties with low turnout.

The chart above shows the total votes, the turnout, and the margin of victory for each county in the state of Texas.  Beto O'Rourke won the five largest counties as indicated by the blue circles from the center to the right, but won them by such a small margin.  Ted Cruz, on the other hand, won the majority of small counties but with really large margins.  Ted Cruz won the counties with both the smallest and largest voter turnout.