I baked some fluid stuff and it was great. But when I saved the file and uploaded it to my online drive, the fluid bake didn't show up and I had to re-bake it again.
How do I keep the bake so that I can upload the file and still have it once I re-download it?
While i do not know exactly which version you used to bake, it would only make a slight difference. The main problem is, that the .blend file you save/upload does not contain the baked cache used for the animation.
As you can see in the picture above, there is a cache folder where the data needed is stored. That folder contains what you baked and needs to be uploaded as well. Depending on the size/resolution of the simulation it can fast become really big though. While the .blend file saves where you stored the cache, you can always just click on the folder icon and search the folder in case you renamed it for uploading or packed and unpacked it somewhere.
Solution: Save the .blend file and pack it together with the "Cache-Folder" to upload it together. Later you can unpack it and only need to select the right folder when opening the .blend file to skip baking anew.
