MacOS Big Sur Terminal command ‘open’ behaviour changed and I don’t know how to get back to how it was in Catalina

There is a way to do this with

osascript -e 'tell application "Terminal" to do script "exec ssh localhost"'

Replace 'localhost' with the host name to which you wish to connect. You can include a username if the remote name is different from your local username, this would look like

rname@remotey.remote-face.org
\____/\_____/\______________/
 |     |      `- Domain name, you may be able to omit if on the same network as 'remotey'
 |     `- Remote hostname
 `- Specify remote login name followed by '@', can be omitted for same username

This suggestion is a variation on the top answer at https://stackoverflow.com/questions/31524499/open-terminal-from-shell-and-execute-commands

Use exec ssh instead of ssh if you want the window to close after logging out. If you use the 'exec ssh' option and your window stays open, then check your Terminal preferences--under "Profiles" > "Shell" > "When the shell exits:" is probably set to "Don't close the window"

Found the answer to this query so wanted to share.

The problem with the code in question was that it was opening the new temrinal window.

The solution was th prevent it from doing that by running the code in already open window, by mentioning the window 1 in it like this:

tell application "Terminal"
    do script "npm start" in window 1
end tell

We can also use keycode and keystroke technique discussed here: https://stackoverflow.com/q/1870270/953566 too, however that was too lengthy and complicated, but can be useful in someones situation.