I have an old library and I want to have all my photos into the latest library
MacBook Pro 15", macOS 10.14
See this article https://support.apple.com/en-in/HT209528
See this article https://support.apple.com/en-in/HT209528
Photos can have more than one library.
You can open your iPhoto library with Photos, and it will be converted to a Photos library.
Hold down the Option key while starting Photos, and choose your iPhoto library.
Not sure about merging two libraries, though.
This page explains several methods, each of which with its pros and cons.
Don't do this unless you are sure you understand the issues involved. In any case, backup first!
https://blog.macsales.com/40866-rocket-yard-guide-combine-multiple-photos-libraries-into-a-single-library
How can I merge iPhoto Library into Photos
