The fixed Huffman codes are pre-defined. From RFC 1951:
3.2.6. Compression with fixed Huffman codes (BTYPE=01)
The Huffman codes for the two alphabets are fixed, and are not
represented explicitly in the data. The Huffman code lengths
for the literal/length alphabet are:
Lit Value Bits Codes
--------- ---- -----
0 - 143 8 00110000 through
10111111
144 - 255 9 110010000 through
111111111
256 - 279 7 0000000 through
0010111
280 - 287 8 11000000 through
11000111
The code lengths are sufficient to generate the actual codes,
as described above; we show the codes in the table for added
clarity. Literal/length values 286-287 will never actually
occur in the compressed data, but participate in the code
construction.
Distance codes 0-31 are represented by (fixed-length) 5-bit
codes, with possible additional bits as shown in the table
shown in Paragraph 3.2.5, above. Note that distance codes 30-
31 will never actually occur in the compressed data.