How are words assigned in machine language?
LDA gets translated by the assembler to bytes that the machine can interpret.

The contents of these bytes are set by the instruction decoder, and often made to make the decoder as simple as possible. For example some bits may get passed straight to the execution unit so add and subtract differ by a single bit which triggers inverting the second operand and sets the carry in bit. The bits that decide which registers to use would be common to all instructions, stuff like that

The 3 letter acronyms are just shorthands set by whoever was in charge of writing the assembler.