I'm a fairly new mountain bike rider. Doing a mix of on and off trail. What is your opinion on handle bar extensions for new riders Just what types of extensions there are and what's best for new riders.  I mainly do off trail but every now and then I'll go off trail
There are several types of extensions available. One type is molded into the handgrip, these tend to be on the  shorter side. Another type tend to extend out and turn toward the centerline. The finish can be bare metal with a knurled finish or a rubber grip installed. The advantage of the inward turn is it prevents the end from becoming a bayonet in the event of a crash or fall. One of the purposes of handlebar extensions is to allow multiple points to hold the bars to eliminate fatigue. Another purpose on earlier mountain bike was to increase stability when climbing. In my opinion on most modern bikes this is no longer necessary as the changes in gearing and geometry have eliminated the need. There are several downsides. If you have your hands on the extensions they aren't near the brakes. A pretty important criteria for a novice rider. The other minus is the extensions move the shifters and brakes inboard. This can have the affect of a slight change in handling, similar to riding with narrower bars.