You can follow a course online or in a school or college or something. But they are general courses about the general ASIC flow. But there are many factors to consider. Analog or digital? HDL? EDA vendor? Target sector (consumer, automotive, industrial, …). Details are depending on these factors. And usually only an experienced ASIC designer in that particular niche can tell you which of the details in the generic courses are of importance to the niche. I see a lot of shallow courses (and books) which are detailing everything and are really boring to read. And it turns out later only 10% of the book is useful for your specific situation. Working in a design team and learning from the best is the most effective way to learn. IMHO.
Where can I learn ASIC design flow (detail not introduction)?