how can I insert a frame in Dreamweaver 2021? just like in the example I attached.
You can't. Why do you think you need frames?
Even iFrames if you can avoid them hey. I not heard someone mention frames and framesets for years. To see someone wanting to use them in 2021 is a suprise to say the least.
As Nancy has stated - Drop that out of your head ASAP.
HTML5 is your thinking.
CSS Grids is the future but I would urge a caution of compatbility especially because of Microsoft. It will be a bit more viable 2022 I think to use more for a client with a large target audiance for example but in terms of learning purposes go for it.
Frameworks such as Bootstrap to help you form your layout (Foundation is another but I am not a fan of this one) will help you produce your structure and layout of the site as well. (Latest all support Flex)
CSS Grids is the future but I would urge a caution of compatbility especially because of Microsoft.
Nobody uses IE anymore. IE was replaced by Edge.
All modern browsers support CSS Grids as well as Flexbox.
When someone chooses to use an outdated browser, that's their problem. They know they have an inferior browser and their user experience is diminshed by it on ALL websites.
For layout purposes, try CSS grids with this CSS Grid generator
Hope that helps.