You mean the 'squeak' on the very first note? That's a picked harmonic.
You do it by touching the string with your thumb behind the pick, at the same time as you pick the note. The pitch you generate is controlled by how far back or forwards between neck & bridge your hand is placed, and 'how much thumb' you give it. You can do it even on an unplugged guitar, but playing it through a high-gain amp structure will emphasise it, because of the compression.
It takes practise to be able to control it, but eventually you will be able to do it at will.
The rest of the 'hollowness' is Gilmour's style & the native sound of his Strat.
It's typical of keeping your picking hand fairly well forwards towards the neck as you're playing, & front pickup is best for it - however [& it's not too obvious in that particular solo, as he's using a higher-gain amp than he did in his earlier work] but the guitar itself contributes a lot to that particular sound.
I used to own a '64 Strat that could make that sound. I now own a '57 Squier Strat copy… which can't.
After comments - this is a great example of Gilmour's 'hollow' sound; from what appears to be a live version, nothing like the original album track - Wish You Were Here, right from the start of the video.
but it's very much "raw Gilmour". You can also hear how the 'hollow' disappears when he switches to the middle [I think*] pickup at 3:20.
[*I think it's middle, because if that's the back pickup it's the sweetest one ever mounted on a Strat ;))
