Open interest information tells us how many contracts are open and live in the market.
If we summarize Zerodha Varsity in few lines in bullets -
Volume on the other hand tells us how many trades were executed on the given day. For every 1 buy and 1 sell, volume adds up to 1
As simple as that.
This is the calculation of OI and Volume. Nothing more is there to it. Now, we assume, if there is a rise in volume and rise in price and rise in OI its bullish.
Open Interest is the property of the contract. Futures of each expiry and Options of each strike price of each expiry has different Open Interest.