What do I wear underneath a trisuit?

(I hope I chose the right flair, I wasn't sure about it. Sorry, if I did something wrong.)

Hello,

I will soon do my first triathlon. A friend of mine was kind enough to lend me one of her old trisuits. It fits me like a glove, the compression is nice. It has a front zip and I asked her what she wears underneath, and she said, "well, nothing."
It made me wonder... I would maybe like to open the zip a bit if it's hot outside, and I would not like to open it while not wearing a bra underneath, haha. I searched some through the internet, but most pages were like: "you don't really wear anything underneath a trisuit."

Do I just have to push through, or is there any kind of underwear I can wear underneath? Sorry if this is a weird question, I am just really insecure and my chest is a bit bigger, so I would like some extra support I think. Thank you for your input!