they are not. you can do cross-messaging between FB and IG if both parties have accounts linked, but otherwise you can have an IG account that is not linked to FB.
Instagram.com tries to execute a script from Facebook.com.
So your data is almost certainly linked.
The origin of a script of unknown functionality to be executed on the client can only tell you so much about the linkage of accounts on the backend.
It tells the site owning the endpoint at which the script it hosted your current IP address and in most instances the site you visited (referrer).
Large internet companies can combine this information across other properties / sources to have a very good idea of who you are.
Internet companies of all sizes can do all sorts of things to identify you. Instagram could also do things server side to link your accounts without any scripts from other origins. The fact a script from FB.com is loaded tells you zero information about account linkage.
By that logic any company including FB ads is a Facebook company
Linking accounts is optional, and combined cross account data is used when they are linked. Otherwise, subsets of the apps may or may not "know" about the link between your accounts via your phone number, email address, or access patterns.