as a composer i never took under consideration harmonics
You might not have been thinking in terms of harmonics, but if you have ever considered which instrument you wanted to use to play a particular part, you have been (indirectly) considering harmonics. The reason that different instruments sound different is that they have different harmonic structures, which give rise to the different timbres:
you can see the different harmonics as the peaks in the 'spectrum' plot on the right.
I read that composers and orchestrators can notate in a way that intends for harmonics to be heard
It is possible for some instruments to play a note in such a way that a subset of the harmonics in the instrument's normal harmonic structure are sounded, and this can be notated - e.g. for violin, or guitar. Playing a note in this way is often referred to as 'playing a harmonic', but that is a different usage of the word 'harmonic'.
Do you learn all notes and what harmonics they produce so harmonics will not collide and sound dissonant in a composition.
Most people learning music in the conventional way don't have to do that. Instead, they use pre-existing musical scales that have been developed over the years and their associated 'rules' of harmony. In Western music, these scales have been defined to suit the partial structures of most Western instruments, which tend to approximately follow an ideal harmonic series. Cultures that have instruments whose partial structure does not follow the harmonic series (for example the Javanese gamelan) tend to use different scales.
Of course if you are actually making a new scale or tuning for your piece (or refining an existing one), you may well be considering how individual harmonics are interacting with each other, as when thinking in terms of limits.
Do you score in the purpose of heard harmonics and if so how or where to learn this?
The 'purpose' isn't really scored in, but how to play the instrument so that it produces the desired harmonics can be (see violin and guitar examples above).
Most of this answer is talking about conventional western composition. If you are doing computer music, you can potentially have complete control over each and every partial in the piece - but it might not be useful to show that using a conventional score.
