It looks like in the object properties it set all scales to 1. Does apply scale just mean setting those properties to 1 or is it more complicated?
Here's a bit of info on that. https://artisticrender.com/how-and-why-do-we-apply-scale-in-blender/
if you scale an object, for example by 100 the object is now 100x larger than it was... when you apply the scale it now assumes that is the new size and puts a 1 in the x, y, z to show that it is now at the appropriate size of 1x... the scale must be applied for some functions to have their proper effect...
hope this helps
Great article linked by @charmn-one and a good response from me1958424. The way I think of applying scale (and transformation / rotation) is establishing new standard transformations of an object.
Take for example an earth model. If I were to create a default sphere in Blender then my "earth" would only be 2 meters across. In order to be an appropriately sized earth I would have to scale my sphere up by a factor of 3.0675 million. Meaning I would type 3067500 in the X, Y, and Z scale values.
However Blender technically still thinks that earth model is a 2 meter sphere scaled up, which is a false premise. This is why a lot of operations in Blender go wonky when scales aren't applied as they should be. The true premise is that the earth is *actually* that big by default. So I would want to apply the scale which tells Blender that my earth model is *actually* that big, making the scale values 1,1,1 while leaving the sphere the same size. This is where the dimension values are handy for differentiating scale and dimension.
I hope that's a somewhat concise / practical explanation.