MVCC - The atomicity problem of consistent reads

[Y M]

Reading and writing to the same tuple at the same time may cause read logic exceptions because of the non-atomic action of writing to the override of the tuple.

For MVCC in MySql,
Conceptually, because of ReadView, access to the tuple being written can be avoided by visibility rules, and then the race between reads and writes occurring in the same region can be avoided
But in terms of implementation details, I still have a question:
Overwriting a field of a tuple will replace the data in the heap. If a read operation comes in, the same area will be read and written, which may result in read and write conflicts (byte copying is not atomic).
How can I avoid this read-write conflict? Is it a lock?