Welcome to the forum!
When we talk about ounces it refers to the thickness of the leather. 1 ounce is .4mm, 2 ounces is .8mm, and you add .4mm for each ounce. So 5 ounces would be 2mm and 10 ounces would be 4mm thick
There are a couple ways to secure leather. Sewing is the most popular, either hand stitching or machine. There are rivets and Chicago screws. Gluing, which is the weakest. we usually glue things together to keep them aligned when sewing.
To do hand stitching you need a way to make the holes; diamond chisels, pricking irons, or an awl.
Using a machine is usually a very high investment, normally in the thousands of dollars. Standard household sewing machines aren't designed to sew leather.
The important question you need to answer is: What do you want to make? If you want to do leather upholstery you need different tools than if you want to make wallets, or horse equipment, or shoes.
Can't really tell what your picture is. If it's a rivet I'm going to guess that it may be a Jiffy rivet.