There are a few ways to figure out what's currently popular. Here are two:
Twitch
Just visit Twitch and peruse the relevant categories. Generally speaking the most popular games (i.e., most players) will also have the largest viewer audiences. You should expect to find Counter-Strike 2, Apex Legends, Valorant taking up the lion's share of viewers but you might find other games appealing. The three I mentioned here are free-to-play like Overwatch 2 and Fortnite.
Twitch has the benefit of capturing multiple platforms (PC, console, etc.) and multiple storefronts (Steam, Epic, Nintendo, PlayStation, Xbox). Of course, your two FPS franchises (CoD, BF) should be well represented at Twitch.
Steam
There are websites that scrape Steam data. Of course, this limited data only covers games on the Steam platform (i.e., PC) and thus ignores other important PC storefronts (like Epic, EA, Ubisoft, GOG, etc.). It also does not capture console gaming. Still you might find this useful. Is Splatoon a FPS? Some might say "yes". In any case none of it will be captured by Steam. Counter-Strike 2 will be the dominant FPS game on Steam since it's the only place you can get it and it has a long established userbase so continue further down the list to see alternatives.
A few other notes
Other ways to find out: there are gaming news sites that frequently compile lists like "best FPS games" or "best free-to-play games."
Many game publishers have transitioned their primary FPS games to GaaS (Gaming as a Service) in recent years so a free download with microtransactions to generate revenue. They use content updates to keep players engaged and the revenue flowing. The modern gamer doesn't like looking at the same map for years and years, they expect new content on a regular basis. Developers don't work for free, servers won't run free either. The ISP also sends a monthly bill.
If you want to play with friends, you are better off just playing what they are playing.
Some of these games have better solo modes than others. Some Twitch streamers actually put the "SOLO" notation in their channel description or tags. Watch those if you are interested in participating solo in multi-player games.
Some of these games are overrun by cheaters. Escape from Tarkov has a reputation for this. Some people have more tolerance to this type of activity than others. The games with a substantial professional competitive scene have fewer cheaters because the game publishers are proactive about hunting down and banning cheaters.
At this point, the TPU community has mentioned about ten games for you to check out. Again, watching gameplay at Twitch should give you some clue as to whether or not you might enjoy it. Escape from Tarkov has garnered a reputation for having pretty accurate weaponry and gunplay. Curiously I've never gotten the feeling that people playing the game are actually enjoying it. In that sense, it's probably closer to real combat in that it's mostly boring nothingness with a handful of terrifying moments. But the latter is typically short since the cheaters will get you. (Disclaimer: I've never playing Tarkov myself, only watched some streamers.)
Anyhow best of luck.
