XPS 13 9360, plugged in all the time or charge it and use it normally?

With lithium-ion batteries, keeping a battery topped up will allow it to last longer than draining and recharging it unnecessarily.  With older NiMH batteries, discharge and recharge cycles were necessary to avoid the "memory effect", but that doesn't apply to lithium-ion, and these different chemistries could explain some of the contradictions you see.

However, you can do even better.  Li-ion batteries don't like to be maxed out on charge level all the time, which is why Tesla cars for example default to only charging to 80% unless you specifically select a Max Range charge, and even that has to be selected every time rather than being configurable as a default.  Additionally, it's good to allow batteries to self-discharge, which means that instead of the AC adapter keeping the battery charged to a certain level, it runs the system off AC while allowing the battery to naturally drain a bit before charging it back up again.  If you go into your BIOS, you'll find you can set maximum and minimum charge levels.  Since I use my laptop at my desk frequently as well, I have my max set to 80% and my minimum set to 50%.  In this configuration, my AC adapter charges my battery only up to 80%, then it essentially disconnects the battery and continues running the system on AC (not the battery) and allows the battery to slowly drain over the next several days rather than keeping it topped up to 80%.  When it has self-discharged down to 50%, the system charges it back up to 80%.  This is the best strategy of all for battery longevity, but the downsides of course are that a) you only ever use 80% of your battery's capacity, and b) depending on when you disconnect your laptop from AC, you might only have 50% charge.  Lenovo has a utility that allows you to click, "Charge my battery to 100% once", which does exactly what it sounds like, so that if you know you'll need to unplug soon and want a full charge, you can do that without disrupting your normal settings.  Unfortunately, I haven't seen an equivalent feature on the Dell side, so if you wanted to charge to 100%, you'd have to change the BIOS settings and then change them back later.