There are countless free/cheap courses online, among others on Udemy or Youtube. Some likely worth it, but I find hard to separate the grain from the chaff. A lot of available Facebook Ads material & courses aren't either recently updated or app specific (you'll find more resources for web/DTC/ecomm/Saas).
A decent starting point is to look at Facebook own free courses "Blueprint" & certifications:
I found those a bit better than their official equivalent for other networks.
Beware of "best practices" preached by those selling ads though...
In my opinion, the best ways to go beyond this base are the following
1-learning by doing. Find a small app, an indie or a friend who's willing to let you play, even if it's not big budget you have to get yours hands dirty. I'd consider proposing yourself for free/cheap for 3 months to advertisers or agencies, and it's very likely you can get a job after that as they're typically more demand than offer in the field right now
2-peer learning: get in communities such as MobileDevMemo or the ASOslack stack (and here!) among others, read & ask. Often you'll get less biased and faster answers than with the support, even more so if you aren't part of a very large advertisers.