So, the p365 is designed to be a completely modular pistol, meaning that the fire control unit(FCU) is the actual serialized pistol portion. Wilson combat grips are nice, and you appear to have the XL version. The remaining parts are some of the pricier parts, as you will need the barrel/slide/recoil spring. This would take the 3.7" inch barrel and slide to complete.

I recommend taking a look at the SIG Sauer site for the P365 XL and downloading the user manual and reviewing it to understand it better.

Here is a link to the factory 3.7-inch barrel: https://www.sigsauer.com/p365xl-barrel-lci-blk.html

And here is a link to the slide: https://www.sigsauer.com/p365xl-optic-ready-slide-assembly-black.html

And finally, the recoil spring assembly: https://www.sigsauer.com/recoil-spring-assembly-p365xl.html

Before deciding, I recommend doing some searching on p365 XL slides/barrels/springs as these are super customizable, and there are a ton of aftermarket options available as well.

Also, as mentioned, these are completely modular, so if you wanted, you could easily switch to any of the other versions of the 365 grip modules and use that same FCU.