How to implement: import an xml file from a third-party resource, and then parse it into the database?

Question

Data transfer

The task is as follows: automatic import of an xml file from a third-party resource, and then parse it into the database to display information in the catalog. So that if any position changes, it changes on the site, if it is added, it is added on the site, and if the position is deleted from the xml file, it still remains on the site.
How to implement it? What are the pitfalls/difficulties?