ABAP 7.4 way of filling one itab from another?

Consider, I have GT_KUNNR internal table like below:

I have second GT_TEXT internal like below:

I am writing the code till now in ABAP as:

LOOP AT GT_KUNNR ASSIGNING FIELD-SYMBOL(<lfs_kunnr>).
  READ TABLE GT_TEXT ASSIGNING FIELD-SYMBOL(lfs_text>) WITH KEY kunnr = <lfs_kunnr>-kunnr BINARY SEARCH.
  IF sy-subrc EQ 0.
    <lfs_kunnr>-text = <lfs_text>-text.
  ENDIF.
ENDLOOP.

My question is, how to achieve above using ABAP 7.4 and upwards? My understanding is, constructor expression (VALUE,FOR,REDUCE..) creates a new type, which I don't need (basically, GT_KUNNR is being updated and can have huge records. So, creating new structure/type and then passing that again to GT_KUNNR will be a performance bottleneck). So, how to achieve that? Any efficient way?

If you want to do this with a constructor operator, then you can use the CORRESPONDING-operator with a lookup table.

But note that this requires that the linking condition between the two tables (kunnr = kunnr) uses a table key in the lookup table. That means that the lookup table (in this case gt_text) has to be a SORTED TABLE or a HASHED TABLE with kunnr as the keyfield, or that it at least has a SECONDARY KEY that matches the condition.

" Declarations:
TYPES:
  BEGIN OF kunnr_struct,
    kunnr TYPE kunnr,
    text TYPE string,
  END OF kunnr_struct,
  kunnr_tbl TYPE STANDARD TABLE OF kunnr_struct WITH DEFAULT KEY,
  BEGIN OF text_struct,
    kunnr TYPE kunnr,
    text TYPE string,
    expired TYPE abap_bool,
  END OF text_struct,
  text_tbl TYPE HASHED TABLE OF text_struct WITH UNIQUE KEY kunnr. " <- Important!

" Test data:
DATA(gt_kunnr) = VALUE kunnr_tbl(
   ( kunnr = '123' text = 'demo text'  )
   ( kunnr = '456' text = 'text inv')
).

DATA(gt_text) = VALUE text_tbl(
   ( kunnr = '123' text = 'welcome' expired = abap_true )
).

" The actual code you asked for:

gt_kunnr = CORRESPONDING kunnr_tbl(
    gt_kunnr FROM gt_text USING kunnr = kunnr ).

My understanding is, constructor expression (VALUE,FOR,REDUCE..) creates a new type, which I don't need (basically, GT_KUNNR is being updated and can have huge records.

This does not create a new type, but according to the documentation I linked above, it does indeed create an interim table with the results in memory. This is unlikely to be a performance problem (the runtime increase would probably be linear at worst), but it could double the memory being used, so it could become a memory problem with very large datasets.

If that is a concern for your use-case, then you can keep using your variant with a loop. If you don't want to use field-symbols, then you can do what many modern ABAP styleguides, like this one from SAP themselves, recommend to use references instead of field-symbols whenever possible. The code from the question using references would look like this:

LOOP AT gt_kunnr REFERENCE INTO DATA(lrs_kunnr).
  READ TABLE gt_text REFERENCE INTO DATA(lrs_text) WITH KEY kunnr = lrs_kunnr->kunnr BINARY SEARCH.
  IF sy-subrc EQ 0.
    lrs_kunnr->text = lrs_text->text.
  ENDIF.
ENDLOOP.