Why do my files uploaded to a Google Drive add a ._ to begining of the file name

It's a sort of sidecar file needed for Mac files stored on some non-Mac file systems (e.g. a  FAT32 USB, NASs, and Online Drives). Its usually small (around 4K or even 0K) and might contain just a file icon or some other information only useful on a Mac. These fragments are automatically hidden on a Mac, but show up elsewhere.

Non-Mac users can usually ignore them, and even if they were deleted, they are recreated on the Mac as necessary... they wouldn't make the file any less usable* on the Mac. Plus, there are ways to hide them on Windows.

*caveat: In older Mac systems, this file fragment was known as the resource fork (since deprecated) and some file types, specifically old Type 1 fonts, would actually store all the font outline information within the resource, instead of the main part (the data fork), so if you, say, copied a Type 1 font to a Fat32 drive, you would see that the "._" file would show with many K of data in it, while the "actual" file name would be 0K, making the font unusable: one of the many reasons Type 1 fonts had to evolve into OpenType. Also:  older pre-OS X graphic files (say an old EPS) would also use the resource fork to store a file preview, but these also could be recreated even if the "._" side was deleted.

Those are, as I dimly understand it, Mac file system index or cross-reference files. (Are they no longer visible/known at the user level?)

I am currently working on a project last done on a Mac a decade ago, and sorting out the real files from this... obsolescent gritware has proven annoying to both myself and the nontechnical author.

Look at the file size--it's probably 1k or 0k (not technically 0, but less than 1). You will usually see two files--your original file and one with the ._ in front of the same file name. This is normally a hidden file known as AppleDouble files and part of the Mac's resourse fork that holds metadata. You usually see them when looking at Mac files on a Windows computers.