How can one acquire merits like mountains in good deeds, equivalent to spending millions, despite not possessing such vast sums?

If your intention is sincere in spending money for the sake of Allah, had you possessed it, then you would receive its reward. The gates of goodness are many and do not require much money, such as glorifying Allah (Tasbih), magnifying Him (Takbir), praising Him (Tahmid), declaring His oneness (Tahlil), enjoining good, forbidding evil, and even lawful marital intimacy. A sincere intention transforms permissible acts into acts of obedience for which one is rewarded.