Since it's forbidden to derive benefit from milk and meat, does that mean it's also forbidden to make a living as a doordash driver who has to deliver things like mcdonalds?
You should ask your Rabbi for a final ruling on this matter. For those wondering what potential issues there might be, and angles for leniency, @Alex posted that Rav Aryeh Lebowitz has a lecture on this exact topic. He discussed the issue with Rav Herschel Schachter, and provided four potential issues, and why they're okay.
The following is just a summary of what he said, and not necessarily an implicit agreement to the arguments provided:
Lifnei Iver - Causing another Jew to stumble. This isn't an issue as it's only a Torah prohibition if the Jew couldn't commit the transgression without you. It's very likely someone else will deliver the food if you don't. There is however a Rabbinic prohibition anyways. Here it would be fine, as the Shach holds there's no Rabbinic prohibition if the Jew doesn't observe any mitzvos whatsoever. We can assume someone ordering non-kosher food isn't a frum Jew with a momentary lapse in observance. Rather, it must be someone, even if Jewish, who isn't observant.
Mar'is Ayin - Doing something that looks prohibited. Walking into a non-kosher establishment could look like you're eating the food there. This isn't a problem nowadays because people would assume you're using the bathroom or getting a drink. Coming out of the store also looks bad, as people see you have their merchandise. However, if you look like a delivery person (you have an uniform/badge/hat that says so, or the establishment clearly displays they use food delivery apps) people won't assume you bought it for yourself.
Business activities with prohibited foods. There's a prohibition (debated if Biblical or Rabbinic) against conducting business with prohibited foods. Rav Herschel Schachter said that delivering food that isn't kosher isn't considered conducting business. The restaurant is; you're just transporting it.
Getting any benefit from milk and meat. This is the biggest issue, as the Mishnah in Avodah Zara explicitly prohibits receiving pay to deliver idolatrous wine, as it's something from which benefit is prohibited. The same should be true here, as milk and meat is also prohibited from benefit. First he suggested maybe it's not Biblically prohibited milk and meat, as the meat wasn't slaughtered properly (neveilah). Rabbinically prohibited milk and meat is permissible to benefit from (Shulchan Aruch Yoreh Deah 87:3; Rema ad. loc. § 1). However, he said the halacha is that even neveilah cooked with milk is prohibited from benefit. Then he suggested that a cheeseburger isn't milk and meat cooked together, as the meat is cooked separately. However, since the meat is probably hot when they put the cheese on it (and it's a davar gush), it's halachically considered cooked together. Also, steak cooked with butter would obviously be an issue. Nevertheless, Rav Herschel Schacter said here it is permitted, as this isn't considered getting paid to deliver milk and meat. You're being paid for delivering the order. No matter what's in the order, you're paid the same amount. Whether it's a $1000 order or a $5 order. So it's not called benefiting from milk and meat.
He concluded that there really should be other job opportunities out there. This isn't necessarily the best job for a frum Jew to take, but would be permissible if necessary.