iPad with virus warning... I'm dissappointed in Big Sur - thought it’d stop this

Break this into three distinct blocks. All of this should be within reach of a novice.

1) Clear the website history.

2) Install the recommended content blocker on both iPad and Mac.

3) Optionally (but recommended), change the DNS settings on your iPad and Mac from Automatic to Manual - and enter the chosen block of four DNS server IP Addresses for your network settings. If anything goes wrong, just change back to Automatic - and restart the device.

Most definitely Scam. Close your browser window - then clear your Safari History & Website Data:

Settings > Safari > Clear History & Website Data

Due to the system architecture of iOS/iPadOS, unless jailbroken, your iPad is not susceptible to traditional malware infection per-se. However, as with all computer systems, there are still vulnerabilities and exploits to which you remain vulnerable.

Browser-based attacks can largely be mitigated by installing a good, trusted, Content and Ad-blocking product. One of the very best and most respected within the Apple App Store - designed for iPad, iPhone and Mac - is 1Blocker for Safari.

https://apps.apple.com/gb/app/1blocker-for-safari/id1365531024

1Blocker is highly configurable - and crucially does not rely upon an external proxy-service of dubious provenance. All processing takes place on your device - and contrary to expectations, Safari will run faster and more efficiently. 

Unwanted content is not simply filtered after download (a technique used by basic/inferior products), but instead undesirable embedded content blocked form download. A further benefit on metered services, such as cellular connections where you data may be capped or chargeable, this not only improves speed but also saves you money.

When using an effective Content Blocker, a high proportion of otherwise inescapable risk when using your Safari browser, or linking to external sources from email, is effectively mitigated before it even reaches you.

A further measure to improve protection is to use a trusted Recursive DNS Service in preference to automatic settings. This can either be set on a per-device basis in Settings, or can be set-up on your home Router. I strongly recommend using one of the following services - for which IPv4 and IPv6 server addresses are listed:

Quad9 (recommended)

9.9.9.9

149.112.112.112

2620:fe::fe

2620:fe::9

OpenDNS

208.67.222.222

208.67.220.220

2620:0:ccc::2

2620:0:ccd::2

Cloudflare+APNIC

1.1.1.1

1.0.0.1

2606:4700:4700::1111

2606:4700:4700::1001

Use of the above DNS services will help to shield you from “known bad” websites and URLs - and when used alongside 1Blocker, provides defense in depth.

I hope this information and reassurance proves to be helpful.

What is important, if using the recommended security focussed DNS provider, is that you should fully substitute your existing DNS servers for that of the replacement. Simply adding the new DNS servers, to your existing set-up, will not benefit from the intended protection offered by using the IP addresses that I have outlined.

Be sure to read this link...

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support