The lore of League of Legends is in an interesting spot. Originally, champions fought as gladiators in a place called Summoner's Rift, under the surveillance of the Institute of War. The idea was that conflict between nations would be limited to this gladiatorial combat instead of using large armies and causing wanton death and destruction. We, the players, were technically in-game characters controlling the champs, with every match being canon (and players are still called "summoners" to this day).
Then, two years ago, Riot decided that this system didn't really allow for a compelling narrative, and retconned everything.
Literally, everything.
This decision still divides the fanbase today. The Journal of Justice was part of this "old lore", and thus is no longer canon.
Ever since the retcon, Riot's writers have slowly been filling out the void with "new lore", faction by faction. Most recently they did Piltover and Zaun, but Shurima, Bilgewater, the Shadow Isles, and Targon also have a lot of new (perfectly canon) lore. Unfortunately there are still a lot of champions who haven't recieved new-lore, and some champion's backstories make no sense at all in the new Runeterra (especially Kayle and Morg).
All the lore of LoL can be found on the Universe website. If a champ's page has a short story and a long bio, it's canon. If the bio is only one paragraph, it probably hasn't been updated for the new lore yet.
Also, this post from Sharjo is a handy summary of all canon events since the dawn of time. This guy knows the lore better than Riot does.
I too found League's lore confusing when I first started reading it. But now I know what's canon and what's not, Runeterra has become one of my favourite fantasy settings. I hope you enjoy it too.