All of my contracts include a full copyright transfer to the client upon full payment. They paid for it after all, and not owning the copyright would put the client in a difficult position if they ever try to raise funding or sell the company.

However, it is common to use open-source libraries when building the project. Some of these libraries are third-party, and some of them may have been written by me.

For simple and quick re-usable components, I tend to rewrite them for each project. For more complex debugging & development helper tools, I write and open-source them in my own time, and include in the project based on the open-source license. If the client is paying for my time, they are not included in billable hours, and if they are paying for results delivered, finding a library to do the job is a great way to achieve these results.