Why is my iPad 8 with iPadOS 18.4.1 showing up as MacOS 10.15.7?

Many websites will identify your iPadOS Safari User Agent as MacOS when used in one of its browsing modes - of which there are two - these being Desktop and Mobile. When operated in its Desktop browsing mode, the User Agent will commonly be identified as MacOS; by contrast, when used in Mobile mode, the User Agent will be identified as iPadOS.

You can verify how your User Agent is being enumerated here:

https://dnschecker.org/user-agent-info.php

It’s easy to switch between modes if a website isn’t functioning correctly. From the desired web-page, touch the aA icon at the left of the Address Bar; this will reveal a menu. From the menu select Website Settings, then select (or deselect) Request Desktop Website, then tap Done. Now refresh the webpage.

The chosen setting will be saved for the selected website - and can be simply reversed if needed.

Frequently happens. If it is a notification saying that an account was accessed on a certain device it will often say it was MacOS rather that iOS/iPadOS. Nothing to be concerned about unless it clearly wasn’t you.

Setting your User Agent to Mobile won't improve your browsing experience. Choosing Mobile optimises your browser for websites whose page layout are principally designed for smaller screens - such as iPhone. While both modes are available, using Desktop mode will properly render webpages intended to be viewed on larger screens.

Seriously, don't get hung-up on how a website identifies and reports your User Agent. This information is merely used, by the website, for page-rendering to your chosen browser.

I've already provided directions on how to change between modes when using the Safari browser. Other browser Apps will likely be using Webkit, this being the underlying system-framework used by many browser Apps on Apple hardware platforms; other browser Apps may have different mechanisms to switch between browsing modes.