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As ASCII character code uses seven bit to encode 128 characters and most computers manipulate an eighth-bit quantity as one byte and ASCII characters are stored one per eight bit, and an additional bit(most significant bit) is taken zero for ASCII characters and other 128 bytes with most significant bit 1 are used for other types of characters e.g. italic etc.

But when even parity is created let's say for example for T (ASCII 1010100) for error detection when transmitting it becomes T( even parity 11010100) with 1 as most significant bit but this code is for another character (from those which have 1 as MSB).

Will it not be overwritten?
How this happens?
Or what I'm lacking in my concept?

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The data bits and parity bit are separate. If you specify 7/E/1, for example, you always get 7 data bits, 1 parity bit and 1 stop bit. – Roger Rowland Oct 19 '15 at 7:24
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Parity bits are added, checked and removed by communications devices (COM ports) at a hardware level.

If you are interacting with a hardware device that expects 7 bit characters, often the interface will be 8 bit and the parity bit will be available on reading, but should be ignored. Using interfaces at this level, will only allow 7 bit characters to be transmitted - and ASCII is a 7 bit code.

If you need to transmit larger character codes - e.g. Unicode (which can be up to 32 bits), there are many encoding schemes like UTF-8 that specify how larger numbers will be broken down into smaller values which can then be transmitted by the hardware.

An example scheme to transmit 8 bit codes over a 7 bit interface, breaks the 8 bit characters into multiple 7 bit characters for transmission over the hardware. e.g. values 1-127 may be transmitted as is, 128-255 may be transmitted as 0 followed by 0 to 127, and 0 transmitted as three 0's

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