I only ask because I wish to learn. When I look in my grammar books, they all say: "When using 'or', if both elements are singular, then the verb is singular too. However, if one of the elements is plural, then use a plural verb."..this copy-paste is from Grammar-monster..so I am slightly confused.
Nope, 'grammar-monster' is wrong on this one, if that is what it says. What are 'all your grammar books'? I suspect you might be misreading them rather than that they are all wrong on this point.
For instance, Swan's Practical English Usage, the nearest grammar to hand and the most generally respected popular text among EFL teachers, clearly states, 'When two subjects are join
Well that is a copy paste, so it is what it says. I otherwise use the compendium from my uni and Strunk's Elements of Style, although that one says nothing on th point, and the intarwebz, in this case I fell on Grammar monster in first hit..so I admit I may have been hasty and gullible. Certainly I have a newfound distrust of Grammar monster. In any case, I did not mean to contest your knowledge,