In the early years of a loan, a much larger portion of each EMI goes toward paying interest, with only a small slice reducing the actual principal you owe. As the loan matures, this gradually flips — later EMIs put a larger share toward principal and a smaller share toward interest, because interest is calculated on the remaining outstanding balance, which shrinks over time.
This is why the total interest paid over a loan's life is so heavily front-loaded, and why prepaying a loan in its early years has a disproportionately large impact on total interest saved compared to prepaying the same amount later in the tenure — you're cutting away principal precisely when the most interest would otherwise have accrued on it.
The EMI formula itself is fixed mathematically based on principal, interest rate, and tenure, so it doesn't change month to month unless the rate itself changes (for floating-rate loans) — but the underlying split between interest and principal within that fixed payment shifts every single month.