First, most of the technologies currently being applied for patents are combination invention technologies, which means that they will include some existing technical features, but will not affect patent authorization.
Second, whether a patent can be granted depends on whether the overall solution composed of multiple technical features is innovative enough, and it does not matter whether one of the technical features already exists.
Third, for the improvement of existing technology, if the improvement is small, you can apply for a utility model patent, if the improvement is large, you can consider applying for an invention patent, but the difficulty and risk of granting an invention patent will be much greater.