do you believe enlightenment is purely esoteric/mystical or can it be concrete in terms of realization and actualization? In other words can enlightenment be achieved by doing x,y and z, or if it is more subjective how does one know they have truly achieved enlightenment?
Not sure what you mean by 'esoteric' or more 'concrete'
It's a spiritual term which comes from spiritual practices. The phenomenon itself is obviously spiritual ie: esoteric the result of which is very concrete. If you take historical accounts of zen masters for example then you could say that after contemplating a zen koan given to you personally by one such master then whilst doing very concrete in life things you may notice something and become enlightened.
Apparently that wasn't such an unusual thing in some parts of the world once upon a time.
Personally I prefer the term path towards.
How does one know when they have reached enlightenment?
One knows when they have achieved understanding to do with riding a bicycle. No more resistance, fear, confusion, trepidation, blame, regret etc. etc. just oneness and ability and enjoyment with a bicycle. It took theory (data, subjective) and practice (concrete action) to achieve.
Lots of mini enlightenment's