How to free RAM in iPhone X?

You weren’t freeing anything when you did that before, btw. The simple act of powering off and back on clears RAM, just as it does on any computer - RAM empties when the device powers off. System RAM is volatile memory.

When on, as mentioned, there is no command or control trick to “clear” RAM. iOS manages RAM very efficiently already, so there’s no point to any such feature.

well closing suspected apps can free up some memory

but they are suspected so they don't use power / battery

when something else need the memory they use they get terminated

terminating them manually use up more power / battery then keeping them where they are until

the memory is needed by something else

Why? There is no need to do so. iOS manages RAM.