Most likely those are crypto ads from Brave browser itself. As per Brave forums and other sites, you can disable these ads.
Disable Brave private ads:
Open Brave browser and go to: brave://settings/rewards
and turn off the toggle for Brave private ads.
As per MakeUseOf: If you want to take this a step further:
Turn off the toggle next to Auto-Contribute and disable any toggle below Tip buttons.
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Disable crypto ads from the new tab page:
There’s one more place Brave will show you crypto ads. On the new tab page.
To remove these ads, open a new tab and click Customize. Then, disable Show Sponsored Images.
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Disable the Brave rewards button:
Go to brave://settings/appearance
Find the hide Brave rewards button and turn on the toggle next to it.
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Additional note: In the comment, you mentioned the primary reason to use Brave is ad-blocking.
In that case, I would recommend to use Firefox with extension, uBlock Origin alongwith powerful blocklist like: oisd blocklist. Also note that oisd is one of many blocklists out there to use with uBlock Origin.
This is how you can add oisd blocklist: uBlock Origin settings > Filter lists > My filters > paste: https://big.oisd.nl/
> Click: Import
.
Reference links:
https://www.makeuseof.com/brave-disable-crypto-features/
https://community.brave.com/t/crypto-notifications/379238
https://community.brave.com/t/brave-crypto-notifications/257295