2nd order Low-Pass Output Impedance and Phase difference
simulate this circuit &ndash; Schematic created using CircuitLab

https://www.electronics-tutorials.ws/accircuits/parallel-circuit.html
The output impedance for your chosen circuit replaces the supply as a 0 Ohm connection ,then you get a parallel RLC circuit Zout=R//XL(s)//XC(s) which has infinite Q with no load.
In reality your SMPS has a switch that when open will have a non-zero resistance and when open may be a different part like a complementary switch or a diode with a different resistance.
But you may appreciate an interactive filter application that runs in any browser using javascripts.
Although very simple , you can choose from Active or passive Filters and  display log gain, Phase, Roots of built-in filters and create transmission line filters.
Real filters have ESR,ESL, and parasitics.
Loads affect the Q and challenge closed loop stability in SMPS.  The other Falstad site allows time domain interactive analysis with custom FETs.  It is also very simple yet powerful with many scope interactive traces allowed.

Below, I switched to Butterworth and enabled Root plots and phase, changed to cutoff @ 200kHz and displayed the attenuation by mouse cursor 12dB down 1 octave up @ 400kHz.

This question could also have been easily answered  with a web search &quot;RLC parallel impedance&quot;  in more detail and less time.  Please learn how to learn.