> Some kind of custom rectangle happening in the top left

I believe that is a "canton": https://en.wikipedia.org/wiki/Canton_(flag)

> In vexillography, the canton is a rectangular emblem usually placed in the upper hoist of a flag, usually occupying up to a quarter of a flag's area. The canton of a flag may be a flag in its own right. For instance, British ensigns have the Union Jack as their canton, as do their derivatives such as the national flags of Australia and New Zealand.

They say it's in the "upper hoist" because it's not per se the top left - If the flag is flying and you're seeing it from behind, it may be the top right. Either way it is the side closer to the flag pole, or whatever it's flying from. When shown on a computer screen or in print, usually it is the top left. Some flag codes require flags to be mirrored on the right side of vehicles or uniforms so that it always appears to be flying from the vehicle or person as they move forward.