Deleting/ Replacing highlighted text issue

I took this from a similar issue I found here but I still having the same problem!

I'm using the MacBook Pro 13' 2015 and an iMac 2019 both Mojave downgraded from Catalina but this has happened since I can remember. I am also a semi new MacUser I was used to Windows way to select and delete Text.

With Windows I was used to:

(Case 1) when I highlight some words and type, the highlighted words would be replaced by the newly typed words, for example

Highlight "text" in "I will check the text" and type "paragraph" would give "I will check the paragraph"

(Case 2) and when I highlight some words and hit backspace, the highlight words would disappear, for example:

Highlight "eating " in "I am eating a donut" and hit backspace would give "I am a donut"

But now with MacOS I have this issue,

in (Case 1), the newly typed words appear all wrong like instead of "text" the first "t" goes at the end and I write "extt"

in (Case 2), the highlighted word cannot be deleted, then sometimes I have to hit "command" before or "fn" I just don't get what is the issue here...

This happens most of the time and sometimes it just works, and it's so annoying!

Does anyone know how to fix this? Did I mess up any settings?