[socrat44 (OP)]

 Einstein's SRT, 1905 - 2022.   
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1905. "On the Electrodynamics of Moving Bodies"/Einstein/
2022. Today we know, Einstein's "electrodynamics bodies" are quantum particles:
light quanta & electrons.
Therefore, Einstein's "On the Electrodynamics of Moving Bodies" is about
the behavior of light quanta & electrons.
#
1905. Einstein asked: "Does the Inertia of a Body Depend Upon its Energy"
The answer was "Yes, the Inertia of a Body depends upon its Energy: E=mc^2''.
2022. Today we know, Einstein's "Body" is quantum particles: light & electron.
It means; inertial movement of light & electron depends on its energy: E=mc^2.
In 1927 Dirac corrected Einstein's writing, E=mc^2 has dualistic nature: E=±MC².
#
''There is one simplification at least. Electrons behave ... in exactly the same way
 as photons; they are both screwy, but in exactly in the same way.''
/Feynman/
Feynman quote means that the electron and photon (as a wave, not as a corpuscular)
 behave in the same way, because they both have energy E=h*f and
difference between them only in frequency (f).
#
"Why do we study so many particles? Understanding really
what an electron is would be enough". /Einstein/
#
You know, it would be sufficient to really understand the electron.
/Albert Einstein/
#
To a request to explain what an electron really is supposed to be we can only answer,
 “It is part of the A, B, C of physics”.
 /Sir Arthur Stanley Eddington/
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An electron is not a simple quantum particle.
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1) Einstein found the energy of electron as: E=h*f
2) Sommerfeld found the formula of an electron as: e^2=ah*c
3) Schrödinger hid the electron in the wave (Ψ) function.
4) Dirac wrote a dualistic formula of electrons:  E=±MC².
5) According to QED, the interaction with a vacuum electron's
energy becomes infinite: E= ∞
6) The electron follows Fermi- Dirac statistics.
7) The electron obeys Pauli Exclusion Law.
The electron obeys the Heisenberg Uncertainty Law (Δx Δp ≈ ħ/4π)
9) Electrons take an important part in Maxwell’s electrodynamics
  (Lorentz force — theory of electrons: F = qE + qv × B)
10) Electrons tied with hydrogen atoms by the energy: E=-me^4/2h*^2= -13,6eV
11) According to SRT an electron cannot be a stable, solid, firm, steel particle.
12) Electrons obey “The Law of Conservation and Transformation of energy/ mass”.
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Conclusion:
We know the electron by what it does, not by what it is.
Electron is much wiser than we are thinking of him.
(but, of course, scientists have the ability to manipulate an electron's free will)
It is better to understand an Electron before discovering new particles.
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[paul cotter]

Science describes the behaviour of matter and energy but what the true nature of these entities is, is something we do not know and may never know.

[Origin]

Today we know, Einstein's "Body" is quantum particles: light & electron.
It means; inertial movement of light & electron depends on its energy: E=mc^2

I don't think this is correct.  E=mc^2 can be used to determine the equivalent energy of an electrons rest mass, not it's KE.
E=mc^2 cannot be used for determining the energy of a photon since a photon has no rest mass.

Einstein found the energy of electron as: E=h*f

That equation is used to determine the energy of a photon not an electron.

According to QED, the interaction with a vacuum electron's energy becomes infinite: E= ∞

An electrons energy is obviously never infinite.

Electron is much wiser than we are thinking of him.
(but, of course, scientists have the ability to manipulate an electron's free will)

Please don't anthropomorphize electrons, they hate that.