Simple said to describe deference between Frame and Forms, is Frames are a sub-Window from main browser, so to handle objects inside we have to switch driver to this window.
But Form is an organizing control to group HTML controls.
see Ref : https://www.w3schools.com/tags/tag_frame.asp
Locate Object in a Frame:
1- Define an object for the frame control.
2- switch the driver to this frame.
3- start finding and dealing with the elements inside the frame.
4- close the frame after finishing.
5- switch the driver again to the default content after closing the frame.
General Example Selenium/C# :
public IWebDriver Driver;
public IWebElement GeneralFrame;
locate your test objects by using findelment functions and deal with them.
after that make sure to close the frame.
then switch to default content.