Hard to hear them because of how the mix is done.

BUT....

They are harmony vocals.... doing the ooooh and aaaah.... you speak of....

They are likely spread wide.... kinda hard to hear because one of the tricks to this sound is the fact that it is NOT very loud and up where you can hear it clearly...and that's intentional......hence the reason lots of folks are missing it.

Quite a few of my songs use this vocal technique because I like it as well.

Main vocal is up the center, and at the highest volume. There are also very likely 2 or more (in multiples of 2) duplicate lead vocal tracks layered considerably lower ....-12dB or perhaps more.... and spread left and right. This thickens the lead vocals....

Then, add harmony vocals or oohs and aahs and have them singing harmony and place them even lower in the mix....

This makes for a really fat sounding vocal where most folks that hear it do not realize that it's a composite of 5 or more tracks.....all at various levels and layers.

The bro country singer has them buried and very hard to hear......so.....

That's different from what you asked on how to get the airey oohs and aaahs.... I think of the Eagles for that sound....and yeah...similar techniques. BUT.... the parts in question are pith perfect, higher in level and with a different reverb than the other vocals... larger space but not maxed out.

Hope that puts you in the ball park for what you're wanting to know.

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www.herbhartley.com
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