Three accounts, three different jobs.

Your bank account holds your actual cash. When you buy shares, money leaves here; when you sell, money lands back here. It never holds shares themselves.

Your trading accountis what your broker uses to place and track your buy/sell orders on the exchange. Think of it as the account that does the transacting - it doesn't store cash long-term or hold your shares either.

Your Demat (dematerialized) account is where your shares are actually held, electronically - the same way a bank statement records how much money you have, a Demat account records how many shares you own, of what, and since when.

When a trade executes, shares move in or out of this account. It's the closest thing to a “locker” for what you own, except nothing physical ever sits inside it.