Freepascal Runtime Library documentation: https://lazarus-ccr.sourceforge.io/docs/rtl/
Lazarus Component Library: https://lazarus-ccr.sourceforge.io/docs/lcl/
Freepascal Component Library: https://lazarus-ccr.sourceforge.io/docs/fcl/
Freepascal Wiki: https://wiki.freepascal.org/Lazarus_Documentation
Something else I should have added in my original comment: Lazarus has a very capable built-in debugger.
That is invaluable if you want to step through and understand how things work.
EDIT: Other sites:
Some demo projects here: https://www.getlazarus.org/
Lazarus project showcase: https://www.getlazarus.org/community/showcase/
The Transmission Bittorrent GUI client was done in Lazarus: https://sourceforge.net/projects/transgui/
Other apps done in Lazarus from the Lazarus website shown by category: https://wiki.freepascal.org/Projects_using_Lazarus