Classic and classical sound like variations of the same word but actually, they are very different.
Classical is linked with a period of time in history. For example, we have the baroque period, the romantic period and also the classical period (1775-1825). So we might describe architecture and art from that period as classical.
It is far more commonly used with music. The orchestral style associated with Mozart, Beethoven or Smetana is called classical music.
‘Classic’ means timeless and is not linked to any specific period or music. A book – like 1984 – can be a classic, or a song like Bohemian Rhapsody can be a classic.
We also use it to talk about typical behaviour from a person. If for example, Honza does something that he is known for doing a lot, you might say it is „classic Honza“.