How and where is a device implanted under the skin by an electrophysiologist to do a 30 day monitor of the heart to check for arrhythmias?

It is not. Placed under the skin but on top of the skin and can be removed for showering .
Ilr. We as electrophysiologists implant these devices called ILR frequently , they can stay with monitoring capability for 3 years not just 1 month.