1. I don't see anything unusual. 2. Some processes will run and use RAM after boot, but settle down once they have completed their routine jobs. e.g. Antimalware is part of Windows Security, updating definitions and scanning for malware. It's normal that it will be quite active for a time after boot. 3. Some applications, such as Google Chrome. are memory hogs. If it matters to you, you could find alternative apps that use less memory. 4. You can disable background services if there are any that you don't need. 5. Unused RAM is wasted RAM. You want the RAM to be used, as that will help your computer's performance. RAM that sits there unused doesn't help anything. Windows will manage when it needs to unload items from RAM to accommodate more urgent processes. So, unless there is some other issue that concerns you, don't worry about it. I don't see a problem. You'll find lots of advice on-line suggesting ways to constrain or alter how Windows manages RAM. Most do more harm than good. Windows 10 does a good job managing RAM. Assuming that you have enough RAM (which you do), Windows will use more when it needs to, and will free up RAM from previous processes as required. I wouldn't worry about it.
Hi, Wondered if anybody could post here how much RAM their XPS 13 Plus using when the laptop is doing nothing, just idling. Mine is about 40 to 45% of 16gb which feels high (?). Only a couple of weeks old and wish I'd checked right after first boot up...now don't know if I've developed a problem already and would rather avoid a full reset to find out.    
XPS 13 Plus RAM used at idle