Throughout your training and career as an electrician you will be constantly assessing current requirements of devices or loads and then sorting out what type of conductor it requires to feed it.
An electrical conductor is a type of object or material that allows the flow of electricity (electrical charge). It has to do with the number of Valence Electrons the material has but your formal training will explain all this.
All material has resistance to electrical charge but due to it atomic structure some material like copper and aluminum have low resistance and other have high resistance. This conductance as we call it is well understood and industry has a vast amount of material to choose from for both conductors and insulators. An insulator is something that impedes the flow of electrical charge.
Generally copper and aluminum are the standard choice of conductors due to their good conductor characteristics and cost. Silver is the best but of course very expensive. Gold does not corrode so is sometimes used in high quality surface to surface connections. Conductivity is not limited to metals and these non-metallic conductors as they are called can be graphite or salts etc.
Electricians constantly have to sort out load requirements (how much electricity is expected to flow through the wire) and then come up with appropriate conductors (wire sizes) that will provide a safe conduit for the electricity. Since this problem has been around since the beginning of the use of electricity there is lots of help out there for electricians.
Finally the electrical codes speak in depth about conductors,
ampacity (the maximum amount of current a conductor can carry before
being damaged), loads etc. Extensive tables with maximum load directives
clearly detail what you can do and cannot use for conductors.