Many scientific articles include supplementary or supplemental materials such as data sets, extended methods sections, additional graphs, etc., that are not required to use to understand the article, but are useful for supporting in-depth review or replication.
Supplements to individual articles are typically free to access and so are not included in the library's subscriptions to journals. If you need to find supplemental materials for a specific article, use a web search engine to find the primary homepage for the journal, then navigate to the volume, issue, and finally the article. You may need to make a personal, free account with the journal itself to be able to download supplementary content.