In the following examples, I'm using [c-] for holding down control while hitting other keys. [c-a,B] means hold down control and hit lower case a and then upper case B. I.e. press control, press and release the 'A' key, press shift, press and release the 'B' key, release control.

vim digraphs in insert mode.

A long list of vim digraphs

Hit [c-k] and then the two characters forming the digraph.
You may need to hold down shift for some.
  [c-k]+-    ±
       DG    °      degree
       <<    «
       >>    »
       e'    é
       e`    è
       12    ½
       oe    œ
       o^    ô
       c,    ç
       a*    α      *a also works
       m*    μ
       p*    π
       w*    ω
       W*    Ω
       2S    ²      superscript
       2s    ₂      subscript
       +S    ⁺
       ->   →
       hh    ─
       HH    ━
       vv    │
       dr    ┌
       PR    ▶
       Tr    ▷
       0u    ☺
       Md    ♩
       M8    ♪
       M2    ♫
Some digraph combinations:
  dr dl    ┌┐
  ur ul    └┘

vim unicode while in insert mode.

Characters using [c-v] followed by three decimal digits.
Only 000 to 255?
  [c-v]177     ±

Characters using [c-v]x followed by two hex digits.
x00 to xff.
  [c-v]xab     «
        b1     ±
        b2     ²

Characters using [c-v]u and then 4 hex digits.
for the unicode.
00000000 to ffffffff?
  [c-v]u2192     →
       u251b     ┛
       u2501     ━
       u2517     ┗
       u253b     ┻

Some border characters using [c-v]U and then 8 hex digits
for the unicode.
  [c-v]U0000251b    ┛