Q1: Where in Rx PHY is the matched filter being used exactly

Not at all / it is a rectangular filter.

802.11a is an OFDM system, and that means it's a multi-channel thing – it divides the wide channel into many narrow channels, each of which is flat, i.e. only a single complex coefficient. It uses a rectangular pulse shape on each of these.

Thus, there's no matched filter explicit anywhere in the receiver.

When you further your study of modulation techniques, you'll learn why OFDM has to use a (in time domain) rectangular filter for each subcarrier (because that achieves the zeros in frequency domain that make the "O" in OFDM).

Q2: If matched filters are used in demodulator as I expect, how is it implemented for a complex demodulation scheme (i.e., 64-QAM).

Filtering is not part of the demodulator, but comes before (in systems that, unlike OFDM, do make use of it).

Q1: Where in Rx PHY is the matched filter being used exactly

Not at all / it is a rectangular filter.

802.11a is an OFDM system, and that means it's a multi-channel thing – it divides the wide channel into many narrow channels, each of which is flat, i.e. only a single complex coefficient. It uses a rectangular pulse shape on each of these.

Thus, there's no matched filter explicit anywhere in the receiver.

When you further your study of modulation techniques, you'll learn why OFDM has to use a (in time domain) rectangular filter for each subcarrier (because that achieves the zeros in frequency domain that make the "O" in OFDM).

Q2: If matched filters are used in demodulator as I expect, how is it implemented for a complex demodulation scheme (i.e., 64-QAM).

Filtering is not part of the symbol decider, but comes before it (in systems that, unlike OFDM, do make use of it), after the downmixing (if applicable).