After reading your original post, I understand that you want to have both Document and Document Set content types in a document library.
Here're steps for you to achieve this.
1.Go to Site settings -> Site Collection Features -> Activate Document Sets feature.
2.Go to Library settings -> Advanced Settings -> Choose "Yes" in the "Allow management of content types?"
3.Also, in the Library settings -> In the Content Types section, click Add from existing site content types -> Add Document Set content type.
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