I started playing Team Fortress 2 for the first time this week. I've owned it since... 2010, when it came with the "Orange Box" (HL1, Portal, and TF2).
Now it's free-to-play.
I'm actually enjoying it. It's not... I mean, it's a deathmatch game. But there's a lot of unique strategy to it. Half-way toward what a MOBA or whatever / overwatch game is today.
For example, a "Heavy". People see the heavy and think it's just for "bros" who want to feel immortal. In fact, newer player tend to scatter at the unintended sight of a heavy.
Except... a heavy is actually not immortal at all. A heavy has a minigun and the most health in the game. But a couple good direct hits, or a single sniper headshot and you're dead! The heavy is also the slowest character by far, and the minigun has to spin up for a second or two before it can fire and even then, your accuracy is reduced the first second of firing (if you don't pre-spin it up). The gun is also VERY inaccurate (and reduced damage) at range.
So everything about the heavy is: Slow to move. Slow to react! Takes a long time to get into place. EXTREMELY sensitive to being ambushed. The gun can't react quickly.
But Heavy + [player with a tactical brain] means you get into a safe place, and take advantage of nooks-and-crannies so that you're cutting down enemies that ideally are not attacking you head-on. You want to cut people down as they pass.
If you get a MEDIC that actually does his job, as he heals people, he charges his "uber charge" meter and once charged, he gets a powerup he can apply. He can have different powerups but the main one is simply IMMORTALITY for him and the person he heals. When a medic "ubers" a heavy, you're a walking tank. But it only lasts for like 5 seconds! So you have to work together to "push" with the entire team.
And that's essential when the enemy team has a very entrenched position with numerous "sentry" fully-upgraded AI-controlled gun turrets.
The other class I've been playing is the Pyro. Again, people say the Pyro is what noobs play because you can just run forward, and shoot. But there's so much more to that if you actually want to be an effective killer. Actual tactics.
The tactical theme is re-occurring: Minimize your classes weaknesses, and maximize its strengths, by choosing where to be, how long to stay, and who to target. (One thing that PUBG has really taught me is... sometimes it's better NOT to shoot someone. Because you may not be able to win a fight, or, that fight will attract MORE people who you then can't beat. So it's important to size up a situation before engaging.)
The pyro has decent speed, a flame thrower and fairly low health. The flame thrower can be incredible. It doesn't do that much damage and has the LEAST RANGE of any class. Many people can literally just run away from you and you'll barely be lapping flames at their heels. But the flame thrower CAN hit multiple enemies in a row. Which means the more people group together, the more you can hit (or even hit them while hiding behind a teammate as a human shield!).
However, the Pyro has a Jetpack (once you unlock it). You can fire it twice (and it slowly recharges), and it has a set timing and flight pattern. You click it and it fires a tiny hop whereever you're looking and then half-a-second-later the main burst fires and you go flying. You feel really cool once you master the timing and direction.
Anyhow, using the Jetpack, you can FLY BEHIND a group of enemies and wreck complete havok on them. One upgraded weapon gives "100% critical hit chance when hitting someone from behind." So you just rip through a group of people all stuck together. They go scattering everywhere!
But again, anyone could outrun you and you have almost NO range for your weapon. So it's all about ensuring the maximum number of people are bunched together before you unleash your flame (or get spotted). You want to create SHOCK AND AWE and then use that second fuel of your jetback to GET THE HELL OUT. Because a group of calm people are going to take you down with a couple of shots.
Another thing is, simply, the mental aspect of it. You can hold off entire groups of people at doorways because they're afraid of being caught on fire--even if the fire won't do "that" much damage. So you can delay them until they eventually get brave enough or take you down. You won't really get points for it but, in the grand scheme of teamwork, any enemies that are focused on YOU are not able to attack the rest of your team, steal a flag, or capture a point.
 I didn't even go into engineers and ammo. Heavy and Pyros run out of ammo fairly quickly! You have to pick up extra ammo from bodies or drops. Or, an engineer can make a dispenser that produces both health and ammo. A good dispenser can basically let a heavy fire non-stop forever! TEAMWORK.
 And the Pyro has a secondary fire mode. An air blast (which takes a ton of fuel). It can reflect enemy projectiles like grenades, and rockets. But I use it to KNOCK PEOPLE OFF MAPS, and, to push medics away from their heavy to separate them and allow my team to take them down one-by-one. (As medics have tons of extra health when attached to and healing a heavy.)