How to set "default" .bash_profile when opening Terminal on Catalina
zsh uses different config files, see the 'STARTUP/SHUTDOWN FILES' section in man zsh. You can run one of
mv ~/.bash_profile ~/.zprofile
mv ~/.bash_profile ~/.zshrc

to rename the file, see the man page or https://support.apple.com/en-us/HT208050 for the difference between these:

If you're using a bash profile, such as to set environment variables, aliases, or path variables, you should switch to using a zsh equivalent. For example:

.zprofile is equivalent to .bash_profile and runs at login, including over SSH
.zshrc is equivalent to .bashrc and runs for each new Terminal session