Besides youtube, there is also a huge amount of documentation to be found about the use of Kicad in the Kicad Documentation you can find here: https://docs.kicad.org/ and also in the FAQ at the Top Right (in the black band) of this forum.
With regards to symbols, there are most of the commonly used symbols in the “Device” library.
For transformers, use the appropriate one if it is present and then re-name it to what you wish.
If it is a footprint you want, you may have to import or create a footprint.
If you need to create or import either footprints or symbols, you will need to create Personal Libraries in which to place those symbols and/or footprints as Kicad Libraries are READ ONLY.
Again, to create Personal Libraries, there are easy to follow (I hope) explanations in both the FAQ here and Kicad Docs here.