She won't need a code, but she will need to accept the support request, i.e., when you connect, a popup wil appear on her screen.

One way to handle that might be to print out a screenshot for her, with an arrow pointing to where she has to press (the "accept") button, and practice with her.

If the device is always at home, consider removing any screenlock, so if she has to restart the device, she won't have to deal with swiping or a password.

Go into the device's settings for Apps - Default apps, and disable those she doesn't need. For example, if she doesn't need Google Assistant, then change the setting for the "device assistant" app to None. Otherwise, accidental random presses, like pressing the Home Button, might result in opening apps she doesn't need, which can be confusing.

I just set up my 101-year old Mom's new Galaxy A71 :), and in fact, I'm still exploring the settings to lock it down. But this is her 3rd smartphone, and she's been using tech for 15 years, so she's kind of a pro. (I'm really proud of her!). Good for your grandma, she will do fine, just be super-patient. Teamviewer works very, very well for us, I can jump in and solve glitches from anywhere.