It appears to be a trend that Systems Engineers say “everything is a system”. While I believe this to be true, I don’t believe that ‘everything’ is a useful definition.
The “true” part is that I define a system as a group (two or more) of entities that exchange matter, energy or information with each other. In a quantum universe, pretty much ‘everything’ meets the criteria of being a part of a system.
However, to be useful, selecting a relevant scope is necessary. I personally focus on systems that are macroscopic, although I recognize that microscopic systems are quite relevant to our daily existence and can impinge on my macroscopic ones. Generally, microscopic level systems and smaller (at least for me) are in the domain of a discipline and there are specialist who address that level of detail. I think about users, not their cells or voltage/current not electrons, although knowing that cells and electrons exist doesn’t hurt. Others (working in medical systems) may need to include cells in their scope .
Why system scope is important
The scope of a system defines its boundary. If you define your scope, it can be determined if your system is a sub-system, super-system or coordinating independent system which aids in the definition of role and responsibility of your system. Interacting with entities outside the system scope indicates needed interfaces and and leads to the definition of required functions.