Resize an image for Facebook

Because they are separate images. A post attaches the picture you upload; a shared link shows whatever the destination page declares in its Open Graph tags, which is usually a wide 1200 x 630 banner set by whoever built that page. If you are sharing your own site, the link card is the one to prepare, and changing it means changing the tag on your page rather than re-uploading here.

Yes. Facebook re-encodes what it receives, which is why uploading a 12 megapixel original gains nothing visible over a correctly sized one and only costs you upload time. Sizing the image first also means you choose which parts get cropped, rather than letting an automatic centre crop decide.

Yes. The image is rebuilt from raw pixels, which drops every metadata block including GPS coordinates as a side effect. That matters more here than on most tools: a photo taken at home and posted publicly can carry the coordinates of your front door, and not every platform strips them.

No, and the page says so beside each number. They are the figures independent references consistently report, plus Facebook's own observable behaviour of re-encoding and downscaling large uploads. Facebook does publish some guidance, but it was not read while these presets were written, so presenting them as official requirements would overstate what we know.

It is enlarged to the target, and the page tells you it did that. Enlarging cannot add detail that was not captured, so the result looks softer than the original - but Facebook would upscale it on display anyway, so doing it here at least lets you see the outcome before you post it.