With respect to the commenters before me, I disagree.
Adding fruit 'flavour' is intuitive and easy. Just add fruit. NEVER use the pith from citrus, though. It will
over bitter a bitter!
One way to use fruit is to give your fermenting brew at least a week to build up enough yeastie alcohol toughness to be ready to devour the sugars from any additive and pounce on any foreign yeast or bugs that might threaten to take over the brew.
Starsan and usual cleanliness procedures obviously withstanding, chop and freeze the fruit in advance, as freezing breaks down cell walls and enables superior penetration. I prefer to rack onto (the frozen) fruit and clean out that first FV's trub.
Secondary ferment times will differ depending on the fruits' fermentable sugar, et al. Flavour continues to deepen beyond fermentation, though. I've left beer steeping on fruit for months to great effect. Strain the likes of mango and peach, which disintegrate into floaties over time. A cold crash will negate that, though.
The sour note is easily achieved with an (initially expensive) addition of GB110 (or another lactobacillus strain). It's a far slower ferment using the likes of that, but it will get there. AND, get there sour!
You can search plenty of cheaper ways to achieve sourness right here, thanks to the great brewers who have come before us.
WAAAAYYY before I went AG, I had a relatively tidy K+K recipe for a Guava Gose. Even the base Gose is worth sharing, if you would like to have it. Sour German wheat, Sea salt, Coriander seed and Guava (or whichever fruit you would like).
Either way, 'flavour' is king!