Search the Internet for reviews of working with a developer. Negative reviews are usually not published on the company's website, but on various review sites you can find the necessary information and find out how diligently your potential developer is working.
If you wish, you can contact his former clients and find out the intricacies of working with the developer: does it meet deadlines, is it comfortable to collaborate, etc.
Hi! It is very important to find professional developers in any kind of activity. It depends on how successful your business will be. So when we needed a new mobile app, we contacted ruby on rails development company JetRuby and were satisfied with the product provided to us.
There are different options on the market. You need first to contact with them and open your ideas, and they will have the image of your goal. For both sides to be on the same page, and to have final products in the form you aimed from start. Maybe flexibility will be even more accepted, again details should be discussed. Try one of the best now in 2020 by ratings and statics on tech-radar, they show that https://www.brainvire.com/full-stack-development/ has the best feedback for the last 2 years. This is what I found in 1 minute, but I consider tech-radar authoritative website.