Hi,

if you are new to odoo development, it is not recommend to directly edit the source code and make changes in odoo. Normally the customization/changes is done using custom module or from user interface using the odoo studio(in enterprise) or using developer mode tools.

If you would like to learn the things, see this: https://www.youtube.com/watch?v=I8FNdellz3Y&list=PLqRRLx0cl0homY1elJbSoWfeQbRKJ-oPO&index=2

Thanks

Start with https://www.odoo.com/documentation/15.0/developer/howtos/rdtraining.html