Broadly speaking an ETF is like a mutual which trades on the exchanges. Expense ratio and trading costs are two different things.
Expense ratio is charged by the issuer (asset management company) Even ETFs have an NAV. It is nothing but the total assets of the fund - liabilities/number of units. Every day when an asset management company declares the NAV, the expense ratio would have already been deducted.
By the way, the price you see on your trading platform is the market price, NAV is different from that. To check the NAV you will have to vist the respective AMC’s website. Here’s an example of Motilal ETFs:
Trading costs are what you incur when you are buying an selling through a broker.