The shortest text heart is <3. Tilt your head to the right and the less than sign becomes the top of the heart, while the 3 makes the two round bumps. It works in any font and on any phone, which is why people still type it next to emoji.
A heart was built into the first PC
The original IBM PC from 1981 came with its own set of 256 characters. Next to letters and numbers it had a smiley, card suits and lines for drawing boxes. The heart ♥ sat at number 3, so holding Alt and typing 3 on the number pad printed it. You can see the whole set on the code page 437 page.
That shortcut still works in many Windows programs today. On a phone, copying a heart from this page is faster.
More hearts you can type
Next to <3, a few other one line hearts are easy to type. A broken heart is </3, with the slash as the crack. Two hearts in a row, <3<3, mean extra love.
The symbols ♥ and ♡ give a full and an empty heart. Some apps turn ♥ into a red emoji heart, so the same text can look different on another phone.
Valentine's Day on February 14 is the busiest day for hearts. Send one to a friend too, not only to a partner. In Finland and Estonia, February 14 is Friends' Day.