How does a person know that what will happen after Salatul Istikhara will be good, and is the questioner's failure to find a job after Istikhara related to his sins, or will Allah choose what is best for him in any case?

If you perform the Istikhara prayer and subsequently do what your heart is inclined towards, then therein lies goodness, even if you dislike what transpired, for its outcome may be praiseworthy. Allah Almighty says: "And perhaps you dislike a thing and it is good for you." (Quran 2:216) This is with an emphasis on the danger of sins and disobedience, and that they are a cause of deprivation and bring about calamities, just as they prevent the answering of supplications. Therefore, hasten to repent and increase in supplication and humble entreaty.