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• yes aaaa writes aaaa forever in what is essentially equivalent to line-buffered mode. The yes utility actually writes multiple lines at a time, but each time it emits output, the output is a whole number of lines.
• echo bbbb; done | grep b writes bbbb forever in fully-buffered mode. It uses a buffer size of 8192, and each line is 5 bytes long. Since 5 does not divide 8192, the boundaries between writes are not at a line boundary in general.

• yes aaaa writes aaaa forever in what is essentially equivalent to line-buffered mode. The yes utility actually writes multiple lines at a time, but each time it emits output, the output is a whole number of lines.
• while true; do echo bbbb; done | grep b writes bbbb forever in fully-buffered mode. It uses a buffer size of 8192, and each line is 5 bytes long. Since 5 does not divide 8192, the boundaries between writes are not at a line boundary in general.

They do interleave! You only tried short output bursts, which remain unsplit, but in practice it's hard to guarantee that any particular output remains unsplit.

• yes aaaa writes aaaa forever in what is essentially equivalent to line-buffered mode. The yes utility actually writes multiple lines at a time, but each time it emits output, the output is a whole number of lines.
• echo bbbb; done | grep b writes bbbb forever in unbuffered mode. It uses a buffer size of 8192, and each line is 5 bytes long. Since 5 does not divide 8192, the boundaries between writes are not at a line boundary in general.

• yes aaaa writes aaaa forever in what is essentially equivalent to line-buffered mode. The yes utility actually writes multiple lines at a time, but each time it emits output, the output is a whole number of lines.
• echo bbbb; done | grep b writes bbbb forever in fully-buffered mode. It uses a buffer size of 8192, and each line is 5 bytes long. Since 5 does not divide 8192, the boundaries between writes are not at a line boundary in general.