How do I transfer App purchased from iMac to iPad

Mac Apps for MacOS - and iPad Apps for iPadOS - are essentially written for different Operating Systems. While some Apps are "universal" these are very much in the minority.

Some App Developers allow you to use both versions of their App (if they exist for the other platform), while some require you to purchase the appropriate version for each.

Apps for iPad are found and downloaded from the Apple App Store. Where Apps that you have purchased are available for both platforms - and do not require additional payment - this will be evident from the download page.