I can give an example rather than explanation. I didn’t realise this part at the time but it seems my home router did not have a perfect connection to the exchange, but it managed OK. Two years later the router was replaced by an engineer as part of a job to fix poor landline function.
Meanwhile my first Android phone, /e/OS, was well connected to the router and using Wi-Fi but was switching to mobile data in a random way and generating Mobile data expenses. Seems mobile data was “filling in” but, inconsistently, Wi-Fi was not always playing the dominant part.
Developer options contains Mobile data always active. Once I switched this off my phone behaved well online. There was no issue that mobile data would not jump in appropriately when there was no Wi-Fi.
Once the router was replaced Mobile data always active was not used.
Apart from this example I believe from my experience Wi-Fi to be prioritised.
What I don’t know, in an environment with excellent mobile 5G would this be preferred over certain Wi-Fi? In an environment with imperfect mobile and / or Wi-Fi does the phone waste energy continually searching for best signal?
A useful list to ameliorate Battery drain on FP6 - #8 by donna1up.