what i need? the first or the full frame equivalent?
Either will do
what the diffrent betwwen them and when i need to use each other?
In photography sometimes focal lengths are described as the equivalent to having a full frame camera (a sensor the same size as the old 35mm film). It’s one way to compare the focal length of lenses when you have cameras with different sensor sizes. You do not have to care about this.
to be a bit nitpicking:
the focal length stays the same, but the Field of View (FOV) angle is depending on the sensor size. so the 3.7mm focal length on the small sensor will give you the same FOV like a 28mm on the full frame sensor.
What it will change is the nature of depth of view. To model camera and lens correctly you need the correct focal length and sensor size, otherwise using right aperture size will give you wrong dof when compared with filmed footage. But from simply technical point of view it does not matter if you scale both focal length and sensor size with some constant, solve will be identical.