core is facades and getConfigData() is a method inside this facades, getCofigData() is used to get data from configuration, first it check that if it exists in database ( which we save from configuration) then it will retrieve from there otherwise from configuration file. core facades is used to get core dependencies of project like currency, channel, locale & many more.
Please check out this guide once: https://bagisto.com/en/how-to-create-simple-product-in-bagisto/
Do let us know if you still face issue in displaying products in front-end.
Regarding products on sale, you can use the special price section or Promotion feature of bagisto which is added in the latest release. You can create cart rules and discount coupons for your customers.
@shreyasi if you want to create navigation menu then you have to override the following php file with your file :
follow the path :
Open '.env' file at root of project & find below one.
Here you may specify the number of minutes that you wish the session
to be allowed to remain idle before it expires.