I have a few questions about Fourier series and filters....

Here's what I understand about Fourier Series:

Any signal (periodic or aperiodic) can be REPRESENTED (key word) as a sum of sinusoidal components (of various frequencies and amplitudes)

Lets consider an arbitrary periodic signal...

Such a signal can be REPRESENTED as a sum of sinusoids having frequencies which are multiples of its fundamental frequency, with various amplitudes ( or "intensity" )

So far so good ?

Here's the part that confuses me..

Let us consider an ideal low pass filter in which this signal is passed through...

The low pass filter will only pass the SINUSOIDS components of low frequency (harmonics) below the cut off frequency.. The result is a NEW signal with only the low frequency remaining components....

This all makes sense...but what I don't understand is the following :

We started off saying that fourier series could be used to REPRESENT an arbitrary signal...but the signal itself, coming from a sensor or any other source, isn't inherently made up of individual sinusoids components summed up together....

so how does the filtering work by removing the "sinusoidal components" if inherently there are none ?

I hope my question makes sense

Thanks guys