Need clarity on vfs-cache-mode full and media unions

Hello, I apologize for not using template as I thought this question is more general.

With vfs-cache-mode full, are writes buffered to disk and queued for upload later? Or are they uploaded right away? I remember this was a feature that was being worked on but I can't figure out if it's happening.

Because I have a union between gdrive and a local directory, if I write files to the unioned directory (P: in my example) does it get uploaded by rclone vfs automatically or do I need to rely on my nightly upload script?

Rclone version Windows:
rclone v1.53.1
- os/arch: windows/amd64
- go version: go1.15

Mount command
mount --cache-dir=c:\rclonecache --dir-cache-time 500h --vfs-cache-max-age 5h --vfs-cache-max-size 10G --buffer-size 64M --vfs-read-ahead 128M --drive-chunk-size 128M --vfs-cache-poll-interval 1m --dir-cache-time 500h --log-level INFO --vfs-read-chunk-size 128M --vfs-read-chunk-size-limit off media_union: P: --config "C:\Users\XXX\.config\rclone\rclone.conf" --vfs-cache-mode=full

rclone config:

[gdrive]
type = drive
client_id = 7REDACTED
client_secret = 6REDACTEDS
scope = drive
token = {"access_token":"yREDACTED","token_type":"Bearer","refresh_token":"1/REDACTED","expiry":"2020-09-28T14:05:09.2388277+08:00"}
root_folder_id = 0REDACTED

[media_union]
type = union
remotes = gdrive: D:\media_union