@4561235 @HighTower @gmacar are all correct.
But to put your mind at ease… the Brave Browser when you opt-in to viewing Ads rewards users by giving BAT (Basic Attention Token) in return. BAT is referred to as a ‘Token’ because it uses an existing Blockchain (network) which is Ethereum.
Bitcoin utilizes its own Blockchain and has no relation to Ethereum so the Brave Browser will never mine (proof of work POW) Bitcoin. For that you would need something like CryptoTab Browser.
As a Brave user for over a year with Ad blocking and Ads enabled I find Brave does not use excessive computing resources (predominantly Brave on Linux). However some others have, see here.