As Tom J Nowell pointed out in the comments, this is a very specific situation, which makes the question rather difficult to answer.
If the source system has suitable code for pushing data to other systems, then you could consider registering a custom REST endpoint at the WordPress end. The endpoint would receive, validate and authorize the POST request from the other system. After passing all the necessary checks the request would then be processed and mapped in appropriate ways to fit the data structure used in WP.
The following functions might be used during the mapping, unless a custom database tables are used. In that case wpdb methods would probably be used.
- wp_update_post()
- wp_insert_post()
- update_post_meta()
- wp_insert_term()
- wp_update_term()
- wp_set_object_terms()
- update_user_meta(), if the data is related to users
Another option is to pull the data from the other system, if it provides an API to do so. In this case you might use wp_remote_get() to request the data.
To schedule the data sync you can use WP Cron. Or if more robust background processing is required, you could consider using the action scheduler library (I’m not affiliated with action scheduler).
In either case, either pushing or pulling the data, the supporing code at the WP end would go into a custom plugin.