X <= not X could actually synthesize into hardware but will properly be less useful for an oscillator as the frequency will depends on internal delay.

But for a simulation oscillator with a after "clock period"/2 statement will it turn out useful.

An other way to utilize  the after statement could be the compunded version where the values and delays separated with commas.

the process of VHDL a quite power tool in order to produce simulation code.

Please note that process will run forever - meaning it will start again automatic when ever it ends.

The only way to "kill" a process will be a wait statement without any time assignment.

The wait for 20 ns will wait for the given time (20 ns)

The Process will repeat forever

A single Wait statement will "kill" the process.