Consider this scene from Saving Private Ryan (1998). Replace Private Stanley "Fish" Mellish with me, a weak female 16 y.o. teenager. I don't even work out or go to the gym!
https://youtu.be/NSRr7wUjLxw?t=183
Assume I'm in the exact same underdog position as Mellish. How can I escape, thwart, and outmanuever this Waffen SS man soldier? I know no martial arts.
Dealing with a knife attack is something addressed in many martial arts. This is not the place to learn this skill, so seek out a teacher to do this in real life. I assume this is academic: you don't plan on actually getting this position. (There are a notable lack of SS soldiers nowadays.)
I'll address this from a Historic European Martial Arts perspective. The best advice is do not get in this situation!
Seriously, going to ground is not in the game plan for these historic systems- stay on your feet and you have a much better chance of survival.
Assuming that fails, there is a general philosophy of "weak against strong, strong against weak" which applies here. The attacker is dedicated ("strong") to the stab, so redirect it (be "weak"). Obviously, oppose the force of the stab so you are not harmed, but focus more on redirecting it away from you. If that knife goes into the (wood?) floor, all the better for you to get out from under them and run away.
Making stuff bend the wrong way when your opponent is extended like this is a classic move, old as dirt and super effective. Here are two instances of doing it after the redirect:
If you luck out and his arm is extended above you, pull the hand with one of yours and push on the elbow so it bends the wrong way with the other. Do this fast and with everything you got to dislocate the elbow.
If the knife ends up on your side, grab that wrist with one hand and pull or push the elbow so the shoulder goes the wrong way. You're looking to dislocate it, so go HARD and FAST.
Do one of these and either the knife drops, your opponent stops attacking, and/or you can squirm out and get away.
This is just one solution, but based on historic sources. (Fiore Dei Liberi)
Every fight is unique and depends greatly on your opponent. It is (trained) reactions to reactions to reactions to circumstances. It is your "lizard brain" handling minor motions while your forebrain handles overall strategy. Go find a class and have fun!
(As has been pointed out by MattM, my earlier comment is probably better placed as answer. I have added some more detail below).
Others may disagree, but I don't think this forum is the right place to learn the necessary skills.
To have any chance at all at coping with such a situation - against a stronger, heavier and determined opponent on top of you (and therefore aided by gravity) - you will require in-person training and lots of highly specific practice. Even then, the chances of coming out unscathed aren't great.
I don't in any way mean to discourage you from learning to defend yourself. There are many disciplines which can enable you to more confidently navigate the world if you're concerned about physical attack.
Explore Brazilian Jiujitsu and Krav Maga and perhaps Muay Thai or Mixed Martial Arts (MMA). Both have utility in regards to the situation you describe, and in regards to many other, far more likely scenarios, as well as providing many other benefits such as fitness, self-confidence and friendship.
Brazilian Jiujitsu equips you with techniques that utilise principles of leverage to enable you to unbalance an opponent in circumstances akin to the one in the clip you provided, and aims to provide practitioners with the ability to overcome stronger and heavier opponents.
Krav Maga emphasises 'street relevant' skills, including techniques designed to combat armed opponents.
Regular Muay Thai/Kickboxing/MMA training will equip you with a wide range of functional strikes and grappling techniques and a high degree of fitness. I mention these because, whilst they aren't strictly self-defence disciplines, they tend to be more widespread than Krav Maga schools.
Even a generic 'self-defence' class by a competent teacher would be very beneficial.
If you can't afford lessons or can't access training facilities for any reason, Youtube contains some excellent videos that may be of some use if you practice the techniques. There is also a lot of junk out there. With this in mind, this playlist isn't a bad introduction to Krav Maga. Some other Stack Exchange Members may be able to provide some better links.
Your best defence will always be avoidance. You should of course never find yourself in such a situation. I understand your question might stem from curiosity alone, but if you're at risk for any reason, seek police advice as soon as possible.
