fundamental astronomy - How did Copernicus get the idea of heliocentric model?

The heliocentric model is historically thought first by Aristarchus. He eventually gave up on the idea due to not being able to give evidence for a concept called stellar parallax: If earth orbits the sun then we should see the closer stars shift in relation to the other stars (strong evidence for this didn't come until Galileo).

Copernicus became to accept the heliocentric model given by Aristarchus due to his study of the Ptolemaic model. His studies showed that planetary motion in a geocentric model was inaccurate.

His model, though, was still based on the fact that "heavenly objects" moved in complete circles. He had to add much complexity to account for this fact which is why the heliocentric model was still not widely accepted. Side note, it was Johannes Kepler, over 100 years later, that claimed that planets move in ellipses rather than circles.