To understand is more to get the grasp of how things work, understand language, understand how financial market works…. To realise is more of a sudden acknowledgment. For example …she was looking for her keys in her handbag to open the door. Only then she realised she left the keys in the office…. If you learn something new ……someone tells you about prices being displayed without VAT in US …….you had no idea so you can say….Oh I didn’t realised that in US the prices are displayed without VAT!

To "realize" often refers to a sudden change or improvement in your understanding. First you didn't understand something, then suddenly you understood it. "I didn't understand why my wife was laughing at me. Then I realized that my left shoe was different from my right shoe."

to understand - to comprehend or to perceive to realize - to become aware / to conceive

Realise tends to mean "know", be aware of. Understand tends to imply that the cause is known, or appreciated, or accepted. . You can also qualify "understand" ot closer to "know" by adding qualification on what you understand. eg I understand that you want this, I don't understand why you want it. . Realise can also be averb, meaning to bring to fruition, to make, to complete. . I realise that you would like to keep a pet, but I have never liked pets, so I don't understand why anyone would have one. . I realise the price has gone up. I just don't understand why.