Create a .bashrc file under ~/.bashrc and away you go. Similarly for ~/.gitconfig.
~ is usually your C:\Users\<your user name> folder. Typing echo ~ in the Git Bash terminal will tell you what that folder is.
If you can’t create the file (e.g. running Windows), run the below command:
copy > ~/.bashrc
The window will output an error message (command not found), but the file will be created and ready for you to edit.