Questions about automotive electronics PCB layout
why they don't design the teardrop in the PCB?


Teardrops are used to reduce the chance of acid traps and perhaps reduce mechanical stress on the trace and via. However, they are not typically necessary. See "Why are there teardrops on PCB pads?" for a more in-depth discussion.


  why they use only one trace connect to the ground?


I'm guessing you mean: why isn't the entire pad connected to the ground fill? This is called a "thermal", and it reduces the chance of uneven heating on the component during reflow, which can cause tombstoning. It also makes it easier to rework, should that be necessary.