Use Intel 64-bit. The AMD Ryzen V1500B is the same architecture.
Note the separate download for DSM 7. Installing the DSM 6 version on a DSM 7 system doesn’t hurt anything, but the install will immediately fail.
When you update to a current version, you’ll see a new shared folder, /PlexMediaServer. This is the Plex Data Folder location, the equivalent of /Plex on DSM 6 systems.
Also, the Synology FAQ has a lot of good info regarding Plex on Synology: Synology FAQ - Questions, Answers, and a few How-To’s
