Android - How to force Google Photos to sync photos to the cloud?

In Google Photos app select Assistant and then you can choose (or view the current backup status/progress) to force backup photos. Be sure to set the different image maps to cloud: the folder will then contain a blue cloud icon with an arrow in it.

Here is what I have found to work most reliably on my phone, which is a stock (not rooted) Samsung Galaxy Note 8 (Android version 9). I found this out via other posts on stack exchange, but I forget where.

After I take a series of photos on my phone using its camera app, I switch over to the Google photos app, and inside that app I select one or more of the photos and create a shared album. That is not enough to sync to the cloud however. To force the phone to sync and upload all the files to cloud, you have to actually share it, or just click on the create link icon. In order for that to work, it forces the phone to have to upload all of the selected photos into Google photos. This works out just fine for me, because I usually want to get a share link to the items that I am taking a record of, anyway

In Android 6 / Marshmallow, go to Settings -> Personal -> Google -> Google Photos Backup, and click Backup all, this started the backup straight away for me. There are lots of other options on that screen that may help, I found that somehow the Backup option at the top of the screen had been set to off.