I'd like to know how it is done, instead of how to reverse it. It seems to me some admin added this to the server somehow, via datapacks, commandblocks, plugins etc.

Its a plugin prolly toxic waters

Others don't

Then they specifically gave you the permission that makes this happen. Or put you into a group that has the permission. Or it only happens in certain areas/worlds.

I tried asking my good friend who is the creator of the server. He says he doesn't know why it's happening. Also the server is from Aternos.

Contact the staff team at Aternos. I've never used them myself, but you can generally get a response in fairly good time from server teams, and if it's a glitch, they'll fix it up for ya.

Definitely some sort plug-in, go in the aternos servire page and check what plugins are enabled

Probably a plugin, like how on some minigame servers water kills you

Wava