Well done,
Though doubtful he learned all the necessary skills just from just YouTube clips, you require a pretty diverse hands on skill set to begin with to undertake all the work involved in a project like this. Also the cost difference is bound to be significant, the quotes from the builders have to take into account all their costs i.e labour, insurance, other trades, certification/inspections required for the services etc... plus the high end price may have come from a builder who was chancing his luck, for example if the building company is really busy and can afford to over price at the first stage then they will take that punt without the worry of losing the job.
I suspect details have been omitted and its been over sensationalised for publicity.
When I bought my first property 30 odd years ago it was a doer upper, just a shell. I did all the work myself (without the advantage of Youtube) but I already had a sound multi skill set base and then just asked advice and figured the rest out myself.
I take my hat off to him though, a man from my own heart.