The people in the know (those who actually beat the market) don't teach other people how to trade, because giving away information is strictly less profitable than trading on it yourself (or on behalf of your firm). They also very rarely trade their own capital, and typically work within funds. This sets up a dynamic where it's extraordinarily difficult to learn the "proper" principles of derivatives trading in a sound framework of market microstructure unless you join a firm, because all public information on the subject is almost definitionally misleading. The next best thing is to learn from established textbooks, but surprise, very few people want to read three - five textbooks with advanced mathematics instead of watching a few webinars on "the greeks."
As the margin required for a spread is substantially less than the full price of the security (which is the whole point of the spread), you can end up in a situation where you're forced to buy more stock than the total liquidation value of your portfolio. Plus this happens at the close on a Friday (as that's when option expiration happens) so you can't liquidate the stock until Monday morning and you're at the mercy of the opening price. If the quantity of options sold is large enough and the price moves against you far enough, you can get completely wiped out.
It's your responsibility to close out your spreads prior to close. FYI, this situation can happen even if they're out of the money as the counter party to the worthless option could still choose to exercise it.
To me, it sounds like you are describing a naked short call, or a short put.
It is not a spread. By definition, spreads have limited risk, and limited rewards.
The four main types of spreads. Take a look at the PNL diagrams. It is limited risk. https://www.optionseducation.org/strategies_advanced_concept... https://www.optionseducation.org/strategies_advanced_concept... https://www.optionseducation.org/strategies_advanced_concept... https://www.optionseducation.org/strategies_advanced_concept...
The situation I’m describing is when the sold option expires in the money but the purchased cover does not. It won’t automatically execute and you’re left with essentially a naked short.
One of the simplest example is to sell a call at $X and purchase one at $X+N[1]. The purchased call is insurance against being on the hook for an unknown rise in the prices of security (which would then be exercised against your sold call). The "bet" in this case is that the rise in the price of the stock during the period of the spread is not going to rise above $X by more than the price difference of the two options.
All the rest of the fancy names are combinations of buying or selling these types of combinations. The names come a combination of the desired result (ex: "strangle") or the shape of the profit graph (ex: "condor").
Do a lot of reading before dipping your toes into options trading. On the one hand it's arguably less risky than regular equities as you "know" what you're risking. On the other hand it's very easy to get wiped out as well. Something as stupid as not closing your our spreads before expiration can destroy your account if they get exercised.
[1]: https://en.wikipedia.org/wiki/Bear_spread#Bear_call_spread