Turning back to the development of pterosaurs to modern birds:
It is believed that dinosaurs and pterosaurs were Ornithodirans.
The dinosaurs went their way on the land and the pterosaurs went their way in the air.
Ornithodira --> Dinosaurs
Ornithodira --> Pterosaurs --> Primitive birds --> Modern birds
The similarities of dinosaurs and primitive birds are symplesiomorphic.
To visualize the idea I am expressing you can think of it this way:
Very early on there was a split of creatures into one line that became land-based dinosaurs and another line that became air-based pterosaurs. Over time, each line developed to adapt to their ecological niche.
The pterosaurs developed through a series of steps to primitive birds (basal paraves) and eventually modern birds.
The dinosaurs on the land developed at the same time on their own line, but eventually died off.
Similarities between dinosaurs and primitive birds are due to their common origin as ornithodirans.
Those similarities are symplesiomorphic.