A few automatic weight / weight painting questions I'm quite new to Blender.

I've been following a few tutorials on Youtube and have made and rigged a human model.

I've merged the armature with its bones and applied automatic weights but barely any of the model has lit up in Weight Paint mode and what has lit up appears to be mirrored on the opposite side of the model which can be seen below.



I don't understand why this bone is having so much influence over the rest of the armature when the whole model is blue.



At this point you might already know what the problem is, but if not here are my thought. The character mesh is being mirrored and I think that at some point I was meant to turn off the mirror, duplicate half of the mesh, flip it and then merge it with the other half. But at no point in the video series I'm watching was this done, so I'm sceptical.

I also can't find how to turn off the mirror. Though I suspect I could if I just googled and while that might fix my issue I'd still appreciate some reassurance or advice.

Thank you.

In the odd chance anyone would like to download this blender file here is a link.
An answer to the second issue: There are different shades of blue and the lighting can hinder your ability to spot the weight paint. What I would suggest is just subtract every part of the mesh to make sure you get it right. And then start painting.