It's kind of long explaination but to be brief
The hypothalamus at the brain is responsible for giving signals to the pituitary gland to start producing Gonadotrophins releasing hormone a.k.a GnRH, this hormone increase the production of Luteinizing hormone resuling in givnig signals to the testicles to produce Testosterone by chemical reactions throw leydig cells
While using Excessive amount of Steroids, it gives negative feedbacks to the hypothalamus therefor interrupts it's job from sending signals to the pituitary to complete the process of boosting/making Testosterone
HCG works as a synthetic Luteinizing hormone which will gives signals directly to the testicles to produce Testosterone but at the same time, being a synthetic exogenous hormone will results in sending negative feedbacks to the hypothalamus, therefor HCG's effects is just temproary but in fact HCG inhibit natural Testosterone production same as Steroids
while Clomid's mechanism, it send positive feedbacks to the hypothalamus by inhibiting Estrogen receptors at this area, therefor the hypothalamus will start working better and as Clomid being a non-steroidal hormone and works by the feedbacks mechanism therefor it will never inhibit the HPTA if compared to HCG