Before the franchise, it was only held in Europe. After the franchise, there are rules.
Champs is held in the league's hub country, and follows the exact one-year cycle of lol worlds venues (22 worlds in US -> 23 champs LA, 23 worlds in korea -> 24 champs seoul, 24 worlds in eu -> 24 champs paris)
25 worlds is in China and 26 is in NA, so it's pretty predictable.
Masters is held in the subregion of each regional league that doesn't host Champs.
M1 is held in an emerging market (Spain is wealthy and one of the centers of European lol esports, but the demand for Valo was very low).
M2 is held in a large country with high demand (Japan)
I don't know exactly how Riot categorizes the subregions, I've categorized them as follows.
Americas: North America, LATAM
Europe: Western Europe, Central and Northern Europe, Turkey
Pacific: Korea, Japan, SEA
China: China
To summarize the competitions held to date
23: Brazil (Americas-LATAM), Japan (Pacific-Japan), USA (Americas-NA)
24 : Spain (Europe-West Europe), Shanghai (China), Korea (Pacific-Korea)
25: Thailand (Pacific-SEA), Canada (Americas-NA), France (Europe-West Europe)
In 26, Champs will be held in China. The regions with longer cycles are Northern Europe, Turkey, and LATAM, so we can predict that the Masters will be held in two of these regions.
On another note, which makes much more sense than my guess, lol esports has already confirmed their long term hosting plans.
2025 FST - South Korea, MSI - Canada, Worlds - China
2026 FST - Brazil, MSI - Korea, Worlds - NA
2027 FST - SEA, MSI - Europe, Worlds - Korea
This is the first year the League has three international events, and Riot is very much in favor of recycling venues, so the plan could be the same every year starting in 26.
The question here is in the case of Korea. SK is the single largest major region in the League. In Valo, it's the hub region of APAC, but it's just subregion anyway. When it's Pacific's turn to host(2028 Champs is a lock), I don't know where they'll host instead of Korea.