A ROM is the operating system on the phone. So you replace all the existing software from scratch.

In a prior comment I just suggested you installed CalyxOS which is a custom ROM. It is very easy and indented for use by non-nerds to provide them with the most private, secure phone which works as expected. You must still be brave to do things you aren't used to perhaps. But it should proceed easily (as long as you get the right phone). Here is the instructions for installing on Pixel 7 from Windows computer as an example https://calyxos.org/install/devices/panther/windows. I was shocked the first time I did it that it worked properly on the first go. I had set the whole day aside assuming I would fuck it up again and again but it was done in like 20 minutes.

When you restart after install, it has everything you need set up: browser, email, chat, app stores, bluetooth works, etc. Just like a regular new phone except it has had as much as possible of the spy tech removed. And unlike some other ROMs it also keeps the device physically secure in case it gets stolen or something.