What theta is

Theta estimates how much an option's value changes as a day passes, with other pricing inputs unchanged. It is an estimate of time decay.

No. The amount changes as expiration approaches. At expiration, extrinsic value is gone.

Use theta as an estimate, not a fixed daily charge, because a change in the stock price or implied volatility can outweigh time decay. Time decay alone does not determine whether a trade makes money. In the practice below, price the same sample call with more days left and with fewer days left.

The stock here is SOFI at $15.42. The numbers are made-up sample data. They are an example and not a recommendation.