This is the purpose of Typst’s named symbols: $ sum_(n in NN) alpha dot n $ produces the expected output, as seen below.
You can see a full list of named symbols (a subset of Unicode) here: General Symbols – Typst Documentation. You can even draw to find out the name of your desired symbol using this website: https://detypify.quarticcat.com/
Note that accessing named symbols outside of math mode requires prefixing them with #sym. (e.g. #sym.alpha). For convenience, you can write as follows to import some symbols from that namespace, avoiding the sym prefix even outside of math mode:
// Without imports:
#sym.alpha
// With imports:
#import sym: alpha
// Output: alpha symbol
#alpha
If a symbol is not in the default list of symbols, you can create your own if you know the codepoint number (or if you just paste the character), by binding either a string or a symbol type (Symbol Type – Typst Documentation) to a variable:
// Simple symbol (no variants)
#let coolarrow = "\u{226B}"
// Just pasting "≫" above would also work
$ coolarrow $
// Complex symbol (multiple variants)
#let letterA = symbol(
"a",
("uppercase", "A"),
("turned", "Ɐ")
)
#letterA
#letterA.uppercase
#letterA.turned
If you don’t have the Unicode character you need but remember what it looks like, there is also a website where you can draw to get any Unicode character: https://shapecatcher.com/