There are a few different ways to go about it. Yes, in THEORY the "cloud" is a possibility but due to the generally (very) large file sizes of video files AND most people's slower upload speeds, it can be very tedious and impractical to do so. Again, possible, but it could take HOURS to upload video files to Dropbox, Google Drive, etc.
What's more practical is keeping your project file and media on an external hard drive. Then you can literally just "plug and play," and open the project on either Mac or Windows. You dont need to do anything special with the project file, just make sure you're running the same version of Premiere on both systems.
Side note, cloud editing IS getting a little easier day by day. If you haven't seen it already, there is a little brother to Premiere coming out soon, codenamed Project Rush. You can learn more here. Project Rush | Adobe video editing software, mobile & desktop
But it's not for everyone and wont have nearly as many features as Premiere. But hope those details help!