While the place where the birds are is inaccessible without assist mode, the beggining can wih the use of a wave/extended hyper dash. The second room in Reflection is possible with the use of wallkicks and yet again wavedashing, as the noticeable wall without spikes to the right of the entrance is a secret area that allows you to use a wave or extended hyper dash
I like how you didn't actually show the solution for the demodash room in farewell because you couldn't do it.(It's a hard room no problem but it made me laugh)
you dont need assist mode for some of these secrets. this video does not show ''how to find'' these secrets the normal way like the title would suggest.
Some Celeste players like doing bingo games against each other, and one common bingo tile is "see birds" or something, so the devs added the room in 3B because there's no birds of that room didn't exist
theres also a secret area in chapter 3 of farewell, in the room where you need to get to the top with the falling platforms covered in springs and spikes, you can get a jellyfish from the previous room and bring it to that room. if you use the jelly to reach the very top right of that room, you can find the car from the prologue
3b Secret room can be entered by using a hyper - onto the big block, then dash around the big block to the other side and then make it The rest you need assist mode