For creating high-fidelity UI designs, we at Codespring use Sketch, a premium application that has been the first choice for designers for many years. We also use Sketch for creating low-fidelity wireframes, but Balsamiq is another viable option for this task.
We use Sketch Cloud for developer handoff, a browser application with a built-in interactive inspector tool. This tool is also great for showcasing our designs to the client so that they can provide feedback on our proposals and we can adjust them accordingly before starting the implementation. Note: The goal is to match these designs during development. 100% fidelity should be achieved.
If custom animations are required for a project, again, Sketch is a great tool for basic prototyping. If these interactions are more complicated, Anima or Principle has all the extra features required for such challenges.
User testing can be done in numerous ways, the digital tools we use most frequently are Maze, Microsoft Forms and in-house review applications. We also conduct personal user testing sessions in-house, if required. All these solutions provide reports, statistics, numeric and descriptive feedback that we can use to improve the product.