https://www.physicsforums.com/showthread.php?t=175438&highlight=cabraham
This issue was thoroughly addressed in the above thread. Read through the entire thread and then we can discuss any questions you have. The crux of my contribution was Einstein's 1905 paper "On The Electrodynamics Of Moving Bodies".
The long and short of it, according to Albert Einstein himself, was that the electric and magnetic fields mutually coexist, and neither is more "fundamental" than the other. The point that some fail to acknowledge is that there are 2 "static" conditions, namely electrostatic, and magnetostatic.
A magnetostatic condition exists with a current carrying loop of steady (dc) value. A magnetic field surounds the wire, but there is zero electric field. This is with respect to the stationary reference frame. To an observer in a reference frame at motion wrt the rest frame (steady velocity, no acceleration), there will be an electric field as well as magnetic.
An electrostatic condition exists when static charges are separated, such as a capacitor charged to a steady (dc) voltage. In the rest frame, an electric field exists, with zero magnetic field. In a moving reference frame, again constant velocity w/ no accelaration, both electric and magnetic fields exist.
According to Einstein, neither the electric nor the magnetic field is the "seat". The term "seat" denoted the "root" or "basis" quantity. In conclusion, electric and magnetic fields are relativistically related.