iPhone
Go to Settings > General and tap About.
Look for IMEI number under the ESIM section at the bottom of the page.
To paste this information, touch and hold the number to copy.
Samsung
Navigate to and open Settings, and then tap About phone or About tablet.
Google (Pixel)
Navigate to and open Settings, and then tap About phone, then click "Model & Hardware" in the menu list and you should be able to find the IMEI number.