How to find all old Libre documents?
What do you mean?
Documents are stored where you saved them. Unless you chose a user locations, there are OS-specific default locations. Since you didn’t mention your OS name, it is impossible to answer your question.
Note this is not an LO-related question. It is a matter of using a computer. If you’re a newbie or casual user, I recommend you read introductory manuals in order to master the basics of file and directory management.
You might care to read this explanation File management - The Document Foundation Wiki.
File management is not part of LibreOffice but is a user operation.