This extension provides TCA information and database tables for the generic "sys_collections" concept, which allows for statically select items into a collection.
In addition, two API classes for fetching items from a collection are available as well:
FriendsOfTYPO3\LegacyCollections\Collection\RecordCollectionRepositoryFriendsOfTYPO3\LegacyCollections\Collection\StaticRecordCollection
Both classes previously resided in the PHP namespace TYPO3\CMS\Core\Collection.
The functionality was part of the TYPO3 Core until TYPO3 v11.0, and moved into its own extension, receiving its own public repository.