For converting triangles to quard faces use Alt+J (Tirds to quards).
For good result in your model, tweek
Max Shape Angle:
About "these faces called": https://blender.stackexchange.com/a/34267/32374
The idea is to decimate the model and the frame shown here is part of
a bigger model and just this is having issue when doing limited dissolve.
About decimating: there is no much difference between this 2 models:
As you see tirs count stays same. That's because n-gons (that created by
limited dissolve) based on same triangles, so there'is no any memory saving in doing this.
Well, the idea is not to triangulate but decimate. This is part of a
larger model and the frame is giving issue
If you realy want to save some memory - use decimate modifier or (much better way) - instancing.