The blur thingy you are speaking about is what we call in photography a "shallow depth of field", which is made usually with a fast lens and/or relatively near the main subject. (I won't enter in detailed explanation)
This was not made artificially (although might have been increased artificially as this picture is already heavily processed)
This the original photo is from pexels.com
You can have/do this effect in GIMP, if your photo does not have it, by separating the foreground (people and area horizontally on the same plane), and blurring different part of the background background with Filters > Blur > Gaussian blur (the Focus blur won't really work in this very picture)
For everything else, rich2005 and Ofnuts gave you extensive explanation, you should just take your time to try - re-try what they told you to perfect the technique until you're happy
Juxtaposing the pictures, you can see what was done, it looks that everything was done ABOVE the original image, thus put your texture above, add a black layer mask to that texture, paint in white on that layer mask starting from the center with brushes explained by rich2005, use different gray > not just black and white to get different effect/opacity on the mask, and you should be good...
Way easier than what you told us what you would like to do
This was not made artificially (although might have been increased artificially as this picture is already heavily processed)
This the original photo is from pexels.com
You can have/do this effect in GIMP, if your photo does not have it, by separating the foreground (people and area horizontally on the same plane), and blurring different part of the background background with Filters > Blur > Gaussian blur (the Focus blur won't really work in this very picture)
For everything else, rich2005 and Ofnuts gave you extensive explanation, you should just take your time to try - re-try what they told you to perfect the technique until you're happy
Juxtaposing the pictures, you can see what was done, it looks that everything was done ABOVE the original image, thus put your texture above, add a black layer mask to that texture, paint in white on that layer mask starting from the center with brushes explained by rich2005, use different gray > not just black and white to get different effect/opacity on the mask, and you should be good...
Way easier than what you told us what you would like to do