Will the files on my external hard drive eventually decay, even if I use the hard drive on a regular basis? What about any hard drive? Thx.?

yes. eventually any HDD will fail somehow. you need to back up files on regular basis

Yes.

"Bit rot" is a real thing, and the larger the hard drive capacity the more bits there are to randomly flip. The hard drive controller keeps most of that hidden from you by detecting and correcting these errors (a few extra storage bits and some math), and when errors accumulate, a drive can mark the sector as bad and use another one in is place. That extends the life of the hard drive but it is not perfect and not forever.

Even so, human error is a more important and common reason to make backups and keep them fresh. If you like your stuff, always have another copy on a separate device.