Follow our tutorial to get a better hang of developing QGIS plugins in QGIS v3.x. The resulting plugin will be capable of querying Nominatim's Reverse Geocoding API from user generated Lat/Long or X/Y in arbitrary coordinate systems and parse the API result into a QGIS Point layer.
A short roundup of Qt Designer, which ships with QGIS and is the recommended program to design Graphical User Interfaces for QGIS Plugins. This post is complementary to our plugin tutorials or just a refresher. It also explains resources.qrc files in more detail, a usual pain point.
In this reference guide, you'll learn more about the intricacies of the signal & slot concept in PyQt. This concept is very important when dealing with GUI development and while it's beautifully solved by Qt, it can be a little confusing in the beginning.