You can check out my work for some examples, but if you have never seen a piece of hand-tooled, hand-crafted leather, you might be in for a surprise. Generally speaking, tooling leather is a lot thicker than garment leather or purse/wallet leather. This is because it is split from the hide in order to be cut into and shaped with tools. The actual thickness of the tooling leather can vary from about 1/16" all the way past 1/4" thick! Because of this extra thickness it is also a great deal more durable and long lasting than the thin, machine sewn leather used to make modern wallets and purses. Also, modern wallet leathers are popularly what I call "pressed leather". Manufacturers take leather fibers and press them with adhesives to make a new sheet of "leather" much like inexpensive bookshelves are made with particle board that is nothing more than sawdust and glue. The thick, tooling leather is almost always only used on the outer parts of an article. A clutch purse, for example will have a tooled leather exterior (about 1/8" thick) with a design literally carved into the surface (much like the rubber part of an ink stamp), and an interior made from thinner "lining" leathers which are thinner than the tooling leather but usually a little thicker, and hence more durable, than what you find in stores.
Any piece that has multiple parts that need to be sewn together will be assembled using one or more of three techniques. For the thinner interior pieces, they are sewn by hand using a professional leather sewing machine (used for shoe repair). This process is always reserved for assembling thin pieces to each other. The next technique of assembly is the saddle stitch. This is done with a waxed thread and two needles weaving their way through the seam to be stitched. This results in a strong, tight stitch between two or more pieces of leather, no matter how thick they are. It's called a saddle stitch because it's the same one used to assemble, you guessed it, saddles! Now you KNOW it's strong! The third technique is the most decorative and possibly the strongest: Leather Lacing. This stitch is done by lacing a thin strip of leather through and around holes punched in the leather. It is used primarily to attach interior parts to the exterior tooled leather. There are many styles of lacing ranging from a simple running stitch that is no more than a single lace weaving through the holes, to the complex "Mexican Basketweave" stitch too complex for words to describe.
I have seen a lot of really old leatherwork and, even with some seams coming loose (results of extreme wear), they are still quite sturdy and durable. Virtually the only way a good leather wallet or purse, etc. will come apart is with a sharp knife!
Feel free to go to my shop to see what kinds of tools I use to make my leather project, and learn about them at the same time!