Historically, most challenges occur in public and school libraries, because that is where the majority of children are able to access books. These challenges are most often brought forth by parents. While these concerned parents have the right to decide what is appropriate for their child to read, they wish to extend that decision and prescribe which books all children can access.
More recently, elected officials and lawmakers in several states have put pressure on public institutions to limit access to books they deem "obscene." This has directly led to the removal or relocation of hundreds of titles from the shelves. Indirectly, it has spurred untold amounts of "silent censorship" where librarians, teachers, and administrators preemptively pull or decide not to order materials that they fear will come under fire.
In 2022, the Georgia governor signed Bill 226 that gives parents the right to report any book for review by the principal, who will immediately remove it from the curriculum. The school then has 10 days to respond to challenges. This supersedes the already established policies districts have for reviewing challenges.
Check out the American Library Association's Guide, Censorship by the Numbers to see which states had the most challenges in 2022 and over the past decade.