I have a guess on satzu and sanayeon based on my observation. So, nayeon and sana actually likes each other since the beginning. But since nayeon is the type who is cautious in relationship and maybe worried it might not go well and it will affect their group relationships, she decided not to pursue with sana. The song Depend on You I feel like nayeon wrote abt her feelings towards sana. See their live MV performance. Only two of them wears purple. They are the main characters. This is so obvious. I was so wow when I wonder whether they told their stylists abt this. So, sana has to move on and then this is when comes satzu. I think sana and tzu might have some relationships together. remember during dahyun's bday live? jihyo said to sana eh we kissed before right. Then, tzu turned and asked sana, you said you never kissed any members before. And all the members were so awkward. mina even signalled to sana with her eyes towards tzuyu. like eh you are going to hurt that girl's feelings. so yeah, I guess they were more than just group mates. Only then maybe recently during the pandemic, nayeon decided to accept her feelings and tend to be more open with sana, hence we see alot of their moments together discreet but quite obvious. As for tzuyu, we seldom see her express herself much but recently since she hangout with chaeyoung and mina alot, I guess she learnt tricks to flirt or they might had advised her some tips. and lately I noticed she is more expressive with sana and we know that's sana's sweet spot, physical bonding. so, now nayeon has to accept that sana is of cos still have feelings for tzu. if you noticed whenever satzu sails during the concert, nayeon face is like upset. but her expression not that too obvious, looks as if she is just looking. maybe that's her style. Anyway, this is just my analysis.