arduino - When should I use opto-isolators and isloated DC-DC converters?

1. I am using multiple DC-DC converters to obtain 12V and 5V. Shall I use non-isolated or isolated converter for Arduino's? From what I have read so far, isolated are used when you have inductive loads and you need isolation in between your motor and the signals which are used to control the motor.


2. Also, Shall I use an opto isolator in between the Arduino and the DC motor,as my Arduino is going to control the DC motor like this:

3. Also do the use of isloated DC-DC converter eliminate the need of opto-isolators , precisely because the ground terminal is separated?

PS: I know my questions previously were vague and I understand the requirements of this forum. I have tried to take care of it this time and I will appreciate if someone can help me with the confusion here. Thanks