Importing master with metric key in glyphs

What happens and how does the merging work:

If i have A.glyphs with 2 masters, and B.glyphs with 1 master
and A has glyphs with layer and/or master keys, and B master also has keys but differently to the masters in A, what happens when I add B into A, does the B glyph keys turn into layer keys?

Or would a glyph in A that did not have a glyph key now have one if the same glyph from B imported has a glyph key?

Depends on how you add B into A.
Did something unexpected happen when you tried?

I do it via font Info, add other master — I did it but haven’t checked carefully, just wondering what Glyphs would do

I see now if I add B into A, B’s glyph keys get overridden with A’s glyphs keys

Metric keys can be stored on glyph level and on master/layer level. Only the second kind is imported when you add a master.