Cross-functional teams are not optimized to work on one component or layer of a system only. This is because:

Cross-functional teams are teams that have all the skills and competencies needed to accomplish the work without depending on others who are not part of the team.

Cross-functional teams are able to deliver value across the entire product, rather than focusing on a specific component or layer. They can work on any aspect of the product that is needed to achieve the Sprint Goal and the Product Goal.

Cross-functional teams are more agile, collaborative, and creative than teams that are specialized or siloed. They can reduce dependencies, handoffs, and delays, and increase feedback, learning, and adaptation.

References:

[Scrum Guide], page 7, section “Developers”

[Scrum Guide], page 10, section “Product Goal”

[Scrum Guide], page 7, section “The Scrum Team”