"What is/are the difference(s) between a strut assembly and a shock absorber?"

The term 'shock absorber' once simply described a standalone part that performed suspension dampening. That is all it did. It had no structural function(the springs held up the car), and was not exposed to lateral(horizontal?) loads (steering/braking/acceleration). It is very common for the rear suspension to have 'shock absorbers' in this sense, like on the 201/124, where the spring and various linkages take on all of the loads, and the shock absorber simply dampens upward and downward travel. Some cars, and even some newer MBs still have standalone shock absorbers on the front.

Typically, I have found that 'strut' is used to describe a structural assembly, meaning an assembly that supports various loads, and the "dampener" or "shock absorber" is one component of this assembly.
The more common setup these days being a "MacPherson Strut", where the shock absorber and spring are all part of a single structural assembly that supports both suspension and lateral loads.
The 201/124 front 'strut' assembly is a bit unusual in that it does not include the spring, so it does not suspend the weight of the car, but it does take on steering and braking loads. The 'shock absorber' part actually does double-duty as the steering pivot point, and rotates separate from its piston when you steer. It also acts as a stop when the downward limit of suspension travel is exceeded.

The terms are sometimes interchanged, or used differently, so it is confusing.
Sometimes the replacement 'shock absorber' portion of a strut is sold as a "strut insert".
For the 201/124, the term "strut assembly" is typically used to describe the replacement part for the front.

Best of luck.

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1986 300E 5-Speed 240k mi.