Standard HTML banner ads like 300x250, 300x600, etc should work with any ad vendor. Google definitely has tools to verify that your code will work properly on their platform and I assume that Sizmek, EyeReturn, etc have them too. Ultimately, your banners have to meet IAB specifications for file size, polite loading, etc. Keep in mind that banners are just a standard HTML document that usually include animation and have well defined borders.
Things get more complicated with rich-media (expandable ads, video players, etc.) because each ad vendor has their own requirements (though not significantly different), templates and support options. I believe all ad platforms support embedding proprietary components like a YouTube player, but you might have trouble getting help from Sizmek, for example, who's Google's competitor.
Google is obviously the dominant player in the ad serving space and luckily their support options are basically free. You can chat with them directly if you have a problem with their products like Web Designer, Studio aka DoubleClick, etc. They sometimes even will write entire blocks of code for you. The big downside with Google in my experience is their documents are not always up to date and you have to dig for information. Unfortunately, I don't know how EyeBuild or Sizmek support their platforms especially when it comes to embedding third-party content like a YouTube player or creating expandable ads.
I've always found changing another developer's project more trouble than it's worth because no two will approach a project the same way. Some will use shortcuts or hacks that will get the job done, but might result in hard to read or follow code. Some don't comment, some write 10 lines of code where 2 might suffice, etc. It's often easier to build an ad from scratch instead of modifying an existing one.
There should be courses out there on Lynda.com or Udemy that are specific to building banners. The ad vendors should also have training or help files. I find that with Google's information is rather limited since they assume you have already built the ad. There is a company called Noble Desktop the runs in-person training and sells books about web development, Greensock, etc.
GSAP does has an official training book on Noble Desktop, but also check out Petr Tichy on Ihatetomatoes.com and his YouTube channel. He has tons of free tutorials on GSAP, React, etc. and e-books you can buy on his web-site. He's a great guy that I've learned so much from.
Hope all this helps.