Can I use other patents when applying for a patent?

You can apply for a patent. At present, most of the patents are the kind you mentioned, which are improved patents. For example, a patent for a water cup may only have a cup body at first, and then someone else applies for a cup with a handle on the cup body, then a cup with a cover, and then the cup body is in double vacuum, that is, heat preservation. This patent, also called subordinate patent, is a patent for further improving the patent or technology of others.