I zip everything up, and use OneDrive with an expiring link and a password. Typically, the link is good for 90 days.

JPEG is a compressed image format that contains less information than the raw files. This is normal.

In Lightroom, click on Export as… then JPEG, size = original (or Largest Available), quality = 100%.

That will be the full resolution image. You can verify the pixel count by right clicking on the image and selecting Properties.