Scrum.org has several free resources, including videos, blog posts, and more, on integrating user experience with Scrum. This represents one perspective from a reputable body. However, it's not the only perspective. I do think there are many ways to integrate user experience with the Scrum framework and what works for one organization may not work for another.
I've primarily seen two main patterns for integrating UX Designers with Scrum Teams.
In one pattern, the UX Designer is aligned with the Product Owner and supports their work in crafting the Product Goal, creating Product Backlog Items, and making sure the Product Backlog Items have sufficient information to be understood by stakeholders. They would typically serve as experts in conducting user research to provide information to the Product Owner, while also maintaining and evolving the design system, planning for A/B testing, and creating any mockups at the right level of granulatity needed to allow the Developers to refine and implement the Product Backlog Items.
In another pattern, the UX Designer is a Developer. They tend to focus on refinement of upcoming work, but may also be involved in the detailed designs and reviewing completed work that impacts the user experience as part of the Definition of Done. In some cases, the UX Designer has sufficient technical knowledge to also be a developer in the software sense and write code related to the user interfaces or that impacts the experience.
Personally, I've seen most success with the first pattern. Especially in a scaled environment where there are multiple teams working on a single product and it may not be economical to either have a UX Designer embedded within each team or ensure that a subset of the teams do work that requires the knowledge and experience of a UX Designer.
By supporting the Product Owner, the UX Designer could be working on earlier concepts and designs and how to evolve the overall design system and information architecture far (perhaps a few Sprints) before the work would be even refined by the development team. The Developers would be able to consume these mockups and proposed changes - which may include concepts from multiple Product Backlog Items - and use them to decompose the work as well as to provide feedback on technical feasibility. The ultimately responsibility to decompose the work would be on the Developers, but the UX Designer would be able to provide guidance on how to evolve the user experience based on how the team can deliver the work.
