copy and paste with middle mouse button

I have encountered this issue for years but I don't understand why.
If I highlight text in say "Firefox" and without clicking copy in the left button menu, sometimes I can just click the middle button in "terminal" and it gets pasted.
Other times I have to highlight the targeted text the copy it then go to the terminal and paste it through the left click menu.
If I highlight text and then click middle button (all within the same program) it seems to work fine, but when different programs are involved highlight then middle button click doesnt do it.
Any advice would be greatly appreciated.
Thanks

There's actually 3 "copy/paste" under X window: Primary, Secondary, and Clipboard. What you are using then in your first example is Primary.

echo primary | xclip -sel p
echo secondary | xclip -sel s
echo clipboard | xclip -sel c

xclip -o -sel p
xclip -o -sel s
xclip -o -sel c

When you select Edit->Copy that's Clipboard. This https://blog.neil.brown.name/2021/07...window-system/ explains the background.

Paste what's highlighted with middle click is part of X11.

Hey, thanks a lot guys. This gives me some new directions to look for the answers.