A simple way to understand this issue:

 - We can legitimately testify about anything we witnesses. This includes
   statements other people make, so long as we see/hear them say such directly.
 - We cannot (in generally) testify about what *other people* witness. If we don't
   see it ourselves, but only hear it from others, that's hearsay (literally, we
   'hear' then 'say').

For example, if a friend tells us she saw a UFO, we witnessed her saying that, and we can legitimately say in court that she said that. But we did not witness her witnessing the UFO, so we cannot testify that she actually did. Only *she* can testify to that.

The 1/6 Panel hasn't (so far as I've noticed) strayed into hearsay. They have accumulated a large laundry list of things that people in and around the previous administration did and said, based on eyewitness testimony of those things being done and said. They are building a fairly damning case based on natural deductions from these statements: e.g., multiple claims from people who witnessed Trump being told there was no credible evidence of election fraud strongly suggests Trump was aware of the facts, so that he is shown to have explicitly and repeatedly lied to the American people. Note the peculiarity that this is not evidence that there was no fraud — that would be hearsay — but it *is* legitimate evidence that Trump was thoroughly told there was no fraud by his own experts and staffers.