With Smart Bidding Google uses all conversion data.
When I audit a new Google Ads account, I frequently find a mix of campaigns, some with Manual bidding and others with Smart Bidding.
Even if an advertiser does not apply Smart Bidding on every campaign, Google will nevertheless use the same data to create the conversion-rate estimate for ads that use bid automation.
As a result, it is critical to assess if all campaigns should be set up with a Smart Bidding approach, as conversions from Manual Bidding affect all Smart Bidding conversions as well.
A typical example.