it works the same as *any* existing application that is compiled for multiple architectures: it uses the current system architecture but default. If you want, you can force an architecture via using "arch" on the command line:
For Pd, you can't do this by double-clicking to launch the application but by invoking the core internally:
arch -x86_64 /Applications/Pd-0.52-2.app/Contents/Resources/bin/pd
This should force running the core as x86_64 with Rosetta 2.
No, nothing has changed about which externals can be loaded. If the external is compiled for the architecture you are running Pd under, then it should load.