Hello, Hannahloove! Thanks for reaching out to Apple Support Communities.
We've reviewed your question and it looks like your Messages app opens to the screen showing the Unknown & Known Messages screen, and you don’t want it to. Allow us to assist with that. Please start by taking a look here: Block, filter, and report messages on iPhone. This article explains more about the Known and Unknown Senders feature.
When you close the Messages app, it should open back to the section you left it on. If this is not happening, please Restart your iPhone and check again. Let us know if this persists. We're here for you. Have a fantastic day!