Enjoy the bike as-is. Drivetrain upgrade in the end might end up costing more than the bike was worth when new. Crankset/chainring/rear der/shifter/cassette and likely a wheel/hub upgrade and that's just off the top of my head. All this on a $100 bike. And at the end of the day it will still weigh a metric ton, lol.
Tires/grips/saddle, pedals, shift cables... as needed, those things make more sense and some can be used on your next bike if you choose. Enjoy the ride. Enjoy learning to ride. Save $$$ so when you decide you are a mtb'er for real, you can upgrade for real. And if not, then spend the money on your next hobby. Or beer.
FWIW I started in '03 with a Trek 4900 (similar to your bike). It was heavy AF, fork was pretty much garbage, had 3x8 gearing, super steep head angle and narrow bars and guess what? I had a blast learning ride. And within a year or so, outgrew it's capabilities. Now, several bikes/frames and $10k or more later I'm still riding. Save that $$$ LOL and hopefully soon that bike will be worth upgrading away from and on to something better/lighter/mor-capable, mor-fun!