Here is my starting point: Raspberry Pi 4, Manjaro - Raspberry Pi 4 KDE Plasma
Unicode special symbols fall into two big categories: the ones showing properly, and the ones displaying just boxes with some pips inside. The trouble is to be seen consistently in Konsole, LXTerminal, and in the i3 status line.
For example (Python, console output):
print("\U0001F53B \U0001f321 \U00002728")
\U0001f321 ("Thermometer") is rendered successfully. I tried different fonts, including [Nerd] DejaVu Sans Mono; nothing seems to make any difference.
It beats me guessing what is the visual difference between the two classes of characters. Certainly none of the wide and colorful is successful :)
The solution courtesy of Dmitry Grigoryev: make sure that the
noto-fonts-emoji packages are installed.