Welcome to the Forums.
Best approach is to search for specific projects to see how others have done it. Pretty much any basic work (and most are on line) about the Pi will go over the basics. Most of what you'll need to know about are the various interfaces, and many of those are so standard that detail descriptions are unlikely. Things like HDMI and USB. There are many tutorials that list the various pins in the GPIO header. If you look around this site, there is a wealth of documentation and (and don't overlook the FAQ and "sticky" posts at the top of the Forum pages). Another place I would suggest looking for good tutorials is Adafruit.
There are relatively few projects that actually call for soldering anything to a Pi, the major exception being adding a header to a Pi0/Pi0W, and a minor one would be adding a 'reboot/restart' button to any current model of Pi. Nearly everything else can be handled with jumper wires and any soldering is off the Pi itself.
Here is a link to help you get started:
https://www.raspberrypi.org/documentation/