This page of the Genshin Impact guide explains what gadgets are, how to craft them and what they are used for.
Gadgets is a new item category recently introduced to Genshin Impact. These items facilitate exploration to a great extent. A gadget may, for example, help you discover chests in your vicinity or create a teleport in a particular place.
To get a gadget, you need to increase your reputation in the city. This is a special reward for achieving a proper level of reputation. Once you get instructions on how to craft it, you have to go to a place with Alchemy or a Forge, use the materials needed to craft the gadget, and then place it in the inventory so that you can use it.