When is energy converted to mass? What are the required conditions to do so? I want to know whether every kind of energy is converted to mass like kinetic and potential energy?
Is it only possible for particles like electrons and protons or even either for big objects like a car or bus.
Please consider the minutest / smallest change in mass even 0.000000000000000000000001 kg of gain in mass?
I mean "relativistic mass" in case when the object is moving and "rest mass" in case when the object is at rest.
Can Rest mass too can increase when the gravitational potential energy of an object increases
 A: I'm going to answer this from the perspective of quantum field theory.
Basically, there are these things called quantum fields which permeate everywhere in the universe. Each field has its own respective particle. You can think of a field like a pond. A ripple in the pond is a particle. In other words, a particle is a disturbance in a field. For example, a disturbance in the electron field is called an electron.
To create a ripple, energy must be used. By 'giving' energy to a field, you get a particle. Some of these particles have mass. Energy isn't really 'converted' into mass.
This answer strays a bit away from your particular need/special relativity, but hopefully, gives a fundamental view of the question.
Here are a few videos on the topic:


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*https://www.youtube.com/watch?v=X9otDixAtFw

*https://www.youtube.com/watch?v=FBeALt3rxEA

*https://www.youtube.com/watch?v=zNVQfWC_evg
A: I'm going to answer without considering any physic, just logic.
Let's say there are 257 types of energy. Let's say one of the 257 types of energy can be converted to mass. 
Now, if we want to convert any type of energy to mass, we can convert the energy to the one type of energy that we can convert to mass, and then we can convert that energy to mass.
