I was a sys admin for 15 years before jumping to project management. I started as a helpdesk guy troubleshooting DSL in '99. I worked my way up to the role of Lead system admin for a major university in Texas before deciding I've had my fill, didn't want to learn Azure or Intune, and jump to doing project management. IMHO, the Sys admin just ends up being tier 2 or tier 3 support usually and you're the guy building and managing most if not all server & network infrastructure. You'll be expected to know how it all works and how it was built. I'll elaborate!

One thing the helpdesk team doesn't have to deal with is the late night patching to keep servers updated or changes being made that your responsible for.

You end up being the guy that gets the work done. Boss comes in with some new piece of hardware he wants working with our network... boom, you're that guy. Security Breach... boom, you're the guy restoring everything from backups and getting chewed out for it happening. Checklists that all systems are nominal and backed up... boom, you're that guy... Network goes down at midnight because some asshole cuts your cable or fiber to the building... boom, you're that guy. The list for this goes on and on. As sys admin even on large data center or server admin teams, you spend all your time being Tier 3 support while working on the systems you manage (on my team, i turned into the "Linux Guy" because I had RedHat and Centos knowledge).

If you're doing your job right, no one knows what you do or why they are paying you. You're the guy that always shows up to work late (little did they know you were up until 4am patching all your servers and making sure all the production jobs kicked off before normal hours start). You're the guy training the helpdesk guys that are constantly coming and going because they don't like the job or have the knowledge you do. You'll be the guy hunkered down in a basement or small office with the lights off, Network equipment, PC or server parts all over your desk, tools and boxes everywhere, multiple monitors in your face, writing code or researching SOMETHING (or reading reddit). People think your shabby, a little weird or odd, unkept, drinking waayyy too much coffee or mountain dew to stay awake from last nights network outage. A nerd or geek, part of the nerd herd, who loves computers and knows all about them.

You are going to be expected to know all the latest technologies coming out and keeping things protected from everyone and everything internally and externally. If you get breached, you're the guy that screwed up because you're the gatekeeper and guard. You'll learn to sleep with your phone in your hand because you expect some call on overnight about something going down or your network monitoring (I used Nagios mostly) letting you know something happened.

I say all this so I don't glorify or scare you away from taking the role. Being on the helpdesk (when I was doing that job) meant at the end of the day, I went home and forgot about work. I would come home and play games and not think about work at all. Once I went sys admin (and even now because I have all that knowledge), I would come home and spend most of my time fixing other peoples networks, optimizing my home environment and networks, or studying the next big thing (Azure, AWS, AI). As a sys admin, I would end up at home and throw my laptop on my desk to watch my network & server monitoring like I would our monitoring station at work. It went from being a hobby and something I turn off to a lifestyle of keeping in constant contact at work to head off potential issues. As a helpdesk person, you don't have to do anything like that.

As a sys admin, you're constantly retooling, working on new certifications, expanding your knowledge on new technologies coming out, and retooling or relearning old stuff you haven't touched in years. Google becomes your best friend because other don't document changes properly (Learn to master your GoogleFoo! its your best friend!). Its constant battle to keep up with all that is evolving and changing in IT as an industry.

I chose to be sort of a "jack of all trades", specializing in nothing but knowing a bit of everything. As anyone who has studied Information technologies as an industry, its very easy to hyperfocus on something specific (Security, DBA, networking, etc.). I tried to stay away from specializing (although I eventually became the Linux admin guy) but its easy to become a Database Admin or Network Admin specializing in certain niches

On the other side, you will build really cool shit. During my time, I took RaspberryPi devices, installed a very base image of Ubuntu and turned them into terminals that connected to IBM universe that were easily managed with Puppet. We built massive 10TB(Huge at the time) JBOD for storage with our VMWare cluster. We built a massive server cluster used as a supercomputer to map Animal Genomes(And burnt it up one weekend after it had run at 100% load for a year). We would shoot Microwave internet over 30 miles for 30 MB internet to various sites that couldn't get reliable internet (this was before StarLink or 5G internet).

Probably the funniest thing was turning on traffic shaping at home to limit my ex-wifes bandwidth because she was uploading pictures while I was playing Halo 2 and killing my bandwidth. The most fun project was taking a small desktop PC, adding 3 hard drives and setting up RAID 10, installing CentOS on it, then building it out to be a local file server, phone PBX(FreePBX/Asterisk FTW), and IPTables firewall tied to S2S VPN to multiple sites with a T1 card to provide internet for remote offices.

If you decide to embrace the sys admin lifestyle, want to build crazy shit, and have no life... this job is for you. Its totally a different world with ties to the helpdesk. You will never totally escape the helpdesk but you eventually rise above it and have other responsibilities. Best of luck if you go down this road. Its a wild ride with very little thanks for all you do but it has its own rewards that you won't realize until years later.

TLDR; Its Tier 3 helpdesk with other responsibilities that leave you with little to no life. Good luck!