There exist some detailed maps for buses in Yangon (example (mirror)). Where can I find similar maps for songthaews in Yangon? I saw many songthaews in Yangon (video example taken near Danyingon railway station), so they seem to be a common means of transportation used by locals in that area.
NOTE: I am not an expert in Myanmar or Yangon, and am writing this answer under the assumption that Songthaews are a form of informal transit operated independently by local people. [Additional note: The word seems to be Thai when googled, not Burmese. The local term may be different.]
There is almost certainly no unified map of the system, and the locals using them most likely know the routes by word of mouth. It is likely that many follow the same route driven exclusively by passenger demand, but the actual routes have no individual identification. To use the system effectively, you will need to speak the language or travel with someone who is native to Yangon, as they will know which routes will take them to specific destinations.
Takeaway: If you're looking for public transportation maps in Yangon, I'd stick with the existing bus maps and bus network since it is likely that no map of informal transportation exists. Buses will most likely be the easier way to get around the city.
General reading on mapping informal transit: https://www.citylab.com/transportation/2014/02/what-informal-transit-looks-when-you-actually-map-it/8283/