It helps to understand which files get sourced when and why.

.profile is sourced by a login shell on startup. Typically, the only login shell you start is the one started when you log in, but you can run a login shell at any time with bash -l. (Also, on macOS, there is no initial login shell, so terminal emulators tend to run a login shell for each new window.)

.profile is an ideal place to set environment variables that can be inherited by any program started from the login shell.

.bashrc, on the other hand, is sourced by non-login interactive shells, such as those started by terminal windows. This is where you set things specific to your interactive shell that aren't otherwise inherited from the parent process. For example, PS1 is set here because only interactive shells care about its value, and any interactive shell will source .bashrc anyway, so there is no need to define and export PS1 from .profile.

And though you didn't ask, it's worth pointing out the difference between .profile and .bash_profile here. .profile is "shared" by all POSIX shells (such as dash), so don't put anything bash-specific here. .bash_profile, though, is only used by bash, so you can use bash extensions in it. If .bash_profile is present, .profile will be ignored, so if for whatever reason you want to use both, you can add . ~/.profile to the top of your .bash_profile.