It's a scam because two things are occurring together:
- It is an all-Internet relationship with no physical presence
- Money transactions are coming into it, and they are cleverly designed to seem harmless
It is a sophisticated scam, and they are getting more sophisticated due to citizens like yourself getting smarter. They can't open with "give me your account info and I'll transfer funds to you" anymore.
But at the heart of it, it's still a classic confidence game. It is about cultivating your confidence in them, so you will relax your guard. Later, they nail you.
I would imagine that 90% of your interactions have been in text, or non-visual non-audio interaction. It is likely that you are being catfished by a foreign man, and the woman is simply "on staff" to show up for the rare video chat, to lend authenticity to all the hours and hours of text conversations.
You can test that by, during video chat, subtly reference a lot of things from text chats and see if she remembers them, or if you're talking to a new person. Or if she fidgets and looks down when you lead a conversational subject, she may be receiving text messages from your actual handler. Also look for an earpiece, headphones (she may be getting a blend of you and your handler) or push-to-talk: all evidence she is getting audio info from a handler.
Another indicator is when her time on video is short or rushed. Ask her to spend a lot of lazy hours on video with you. If this woman is scamming many people, the scam company won't be able to afford having their one woman spend all that time on only one victim. Whereas a real woman would not have any trouble with that.
Be super discreet and playful about these challenges. If they realize they are challenges, they will get very offended and manipulate you to back down (but then a normal person might get offended too, so that doesn't tell you much).
On the financial side, what's happening is they have stolen bank details from a different victim. They aim for you to steal out of that person's account. That's either to get you comfortable with this to put you off your guard for later frauds, or simply, they will fabricate a lie about how they need you to take more than agreed and send some money back to them, e.g. via Bitcoin or Western Union. At that point, it's just a cashiers check scam.
But it's all about building your confidence that they are legit. People like you, who know enough to be on guard, are the most vulnerable to this because if they gain your confidence, you'll trust your own vetting and thus, trust them even more.