What is the dom access containment in salesforce?

A component can only traverse the DOM and access elements created by a component in the same namespace. This behavior prevents the anti-pattern of reaching into DOM elements owned by components in another namespace.

ref: https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/security_dom.htm#:~:text=A%20component%20can%20only%20traverse,by%20components%20in%20another%20namespace.