On my Samsung phone, I can add a symbol "₿" Bitcoin on the Samsung keyboard. If you are using the same device, try to enable the smart typing and the key symbol under "text shortcuts" It might be different on other Samsung models, but look for "personal dictionary" just in case you can't find the "text shortcuts" and then add and rename it BTC or Bitcoin for expanded phrase, put "₿".

Once you've done that, just type "BTC" or "Bitcoin," and it will suggest the symbol "₿". That's my setup on my Samsung keyboard.