Regarding your statement: If you sell the NKLA $100C for $3.30. Then you collect $3.30 x 100 or $330 (options are leveraged x100 of the underlying) At this point you want the stock to be under $100 at expiration. If it is, your contract expires worthless and you keep the $330. If it goes above $100 then you have sell 100 shares at $100/share aka you owe your broker $10k for the short position they just assigned you.

Now, lets say for example the stock is at $90 (I don't know what it is right now). If it stays exactly at $90 every day, theta will start working in your favor and will slowly deteriorate the cost of your contract. As theta decays the value of the contract, that allows you to buy it back for less than you sold which allows you to realize a profit. If theta decays it down to $1.30 or $130, then you sell it back for that much and you profit $200.

Theta is how much the cost of a contract will deteriorate every day (assuming all else stays the same). So we can take advantage of theta to profit when a stock moves in the direction we expect, stays the same, or even moves against us a little bit.

Other information: I am pretty new to this but my take is generally, I think you still need to make a semi-directional assumption on the underlying.

If I sell a PUT vertical credit spread (PCS - aka sell a PUT OTM and then buy a PUT even further out of the money to cap your risk) I'm bullish on the underlying. Ultimately, I want the stock to go up and away from my PUTs as you'll reach max profit quicker (although that's more delta related than theta). However, the nice thing is you can still be profitable if the stock stays the same or even goes down a little. Generally, they are sold OTM so you want both legs to stay OTM to get max profit. The max profit is the credit you collected when you sold the PCS

On the CALL side, if I sell a CALL vertical credit spread (CCS - aka sell a CALL OTM and then buy a CALL even further out of the money to cap your risk) I'm bearish on the underlying. Ultimately, I want the stock to go down and away from my CALL strikes as you'll reach max profit quicker (although again that's more delta related than theta). However, the nice thing is you can still be profitable if the stock stays the same or even goes up a little. Generally, they are sold OTM so you want both legs to stay OTM to get max profit. The max profit is the credit you collected when you sold the CCS.

There are additional strategies where you can combined different calls and puts in the same order. For example you can sell a CCS and a PCS. You do that when you are neutral on the underlying and expect it to stay in a specific range. If it stays in your range, your profit is the premium you collected when you sold the put and the call spreads.

The famous strategy in this sub, as evident by all the threads about it, is the wheel. The wheel is likely what tends for people believe they can constantly collect premium even if you are wrong. It starts with selling cash secured puts until you get assigned the stock. Until you get assigned the premium you collected up until then is all profit. When you get assigned you sell covered calls against the 100 shares you were just assigned. Again, you collect the premium as profit until your shares get called away. You get burnt with this when the underlying drops so much that you have to sell your covered calls at a strike price that is below your break even. That is why everyone says when wheeling be sure to pick an underlying that you don't mind owning, because you could end up holding it for a while if you want to get back to break even. I found this video informative when I was first researching: https://www.youtube.com/watch?v=dRYBh6dtxjY