So Iv'e read the rules and tried to install the 1.15 Version on Minecraft W10 and I saw there is no option to do that in Beta, only on third party minecraft Launcher but the Rules say: ,,Full game runs performed on a beta version of the game should be submit to the category extensions leaderboard'' So how can I play on older Versions of the Game if there is no option to switch like in Java?

There was never was 1.15 version on Bedrock. They have a 1.15 beta but ended up just going straight to 1.16 to be on the same version as java. 1.15 doesn't really have any sort of advantage for speedrunning, it's just 1.14 villager trading with bees lol. The beta CE was mainly a place to put the 1.16 beta runs so the main board categories weren't broken. Old versions from the windows 10 launcher can show up as a beta such as 1.5.3 but is allowed because it doesn't give any advantage.

Ye but its not possible to go back to 1.15.3 bc Bedrock doesnt have any option for that. I have to download a MCLauncher and play there but its against the rules so how can I catch up with those who made it in 1.15.3? Tried the Seed in 1.16.1 and the World is a different one.

There is no 1.15 version

do you mean 1.5.3? or 1.15 i'm confused

1.15 doesnt exist. we use version 1.5.3 but 1.15.3 doesnt exist.

MC Launcher is also allowed to go back to older versions. 1.16 beta runs will be put in category extensions though.

the only version you can physically use on console rn is 1.16 unless you do some weird stuff to downgrade to the earliest version.

the rules dont say its not allowed. you can downgrade.

dear lord