Unicode.

On Linux: press CTRL + Shift ⇧ + U and enter the number below. On Windows, search for the “alt codes”. Symbols in grey may show up as ▯ on Android. List of “all” Unicode symbols.

Operator, relations:

± ∓ | 00b1, 2213
× ÷ | 00d7, 00f7
≈ ≠ | 2248, 2260
≤ ≥ | 2264→5
‰ ‱ | 2030→1
㏑㏒ | 33d1→2
⇒ ⟺ | 21d2, 27fa
√ ∛ ∜ | 221a→c
∀ ∁ ∃ ∄ ∅ ∈ ∉ | 2200→1, 3→5, 8→9
⊂ ⊃ ⊄ ⊅ ⊆ ⊇ ⊈ ⊉ ⊊ ⊋ | 2282→b
⋂ ⋃ | 22c2→3
¬ ∧ ∨ | 00ac, 2227→8
⟨ ⟩ ⟪ ⟫ | 27e8→eb
∝ | 221d

Numbers, letters:

№ | 2116
∞ | 221e
ⅈ ⅉ ℑ ℜ | 2148→9, 2111, 211c
ℇ ℯ ⅇ | 2107, 212f, 2147
ℕ ⊂ ℤ ⊂ ℚ ⊂ ℝ ⊂ ℂ | math symbols wiki
∠ ⟂ | 2220, 27c2
ΑΒΓΔΕΖΗΘΙΚΛΜΝΞΟΠΡ ΣΤΥΦΧΨΩ | 0391→a9
αβγδεζηθικλμνξοπρςστυφχψω | 03b1→c0
𝔄𝔅ℭ𝔇𝔈𝔉𝔊ℌℑ𝔍𝔎𝔏𝔐𝔑 𝔒𝔓𝔔ℜ𝔖𝔗𝔘𝔙𝔚𝔛𝔜ℨ | Wikipedia
⁄ | 2044
⁰ ¹ ² ³ ⁴ ⁵ ⁶ ⁷ ⁸ ⁹ | 2070, 00b9, 00b2→3, 2074→9
₀ ₁ ₂ ₃ ₄ ₅ ₆ ₇ ₈ ₉ | 2080→9
⅟ ½ ↉ ⅓ ⅔ ¼ ¾ ⅕ ⅖ ⅗ ⅘ ⅙ ⅚ ⅐ ⅛ ⅜ ⅝ ⅞ ⅑ ⅒ | vulgar fractions

Units & quantities:

℃ ℉ | 2103→4
㎜㎟㎣㎝㎠㎤㍷㍸㍹ 𝗆 ㎡㎥㎞㎢㎦
㎧㎨ | 33a7→8
㎎㎏㏖ | 338e→f, d6
㎅㎆㎇ | 3385→7
㎐㎑㎒㎓㎔ | 3390→4
㎭㎮㎯ | 33ad→f

Games & probability

♔ ♕ ♖ ♗ ♘ ♙ | 2654→9
♚ ♛ ♜ ♝ ♞ ♟ | 265a→f
♠ ♡ ♢ ♣ ♤ ♥ ♦ ♧ | 2660→7
⚀ ⚁ ⚂ ⚃ ⚄ ⚅ | 2680→5

Arrows:

← ↑ → ↓ ↔ ↕ | 2190→5
↖ ↗ ↘ ↙ | 2196→9
⇠ ⇡ ⇢ ⇣ | 21e0→3
⇦ ⇧ ⇨ ⇩ | 21e6→9
⇽ ⇾ ⇿ | 21fd→f
↰ ↱ ↲ ↳ ↴ ↵ | 21b0→5

Ballot box:

☐ ☑ ☒ | 2610→12
✓ ✔ ✗ ✘ | 2713→4, 7→8
⍻ | 237b

Misc:

… | 2026
< > < > | 202f, 00a0
< >< > | 2002→3
– — ― | 2013→5
"" '' “” «» | 0022, 0027, 201c→d, 00ab→bb
† ‡ | 2020→1
‽ | 203d
☺ ☻ ☹ | 263a→b, 2639
❦ ✌ | 2766, 270c
☀ ☁ ☂ ☃ ☄ | 2600→04
⚠ ☠ ☢ ☣ | 26a0→20, 22, 23
☭ ☮ ☯ | 262d→f
⚔ ⚕ ⚖ ⚗ ⚘ ⚙ ⚚ ⚛ ⚜ | 2694→c
🯅 🯆 🯇 🯈 🯉 | 1fbc5→9
🔒 🏞 🌪️| | 1f512, 1f3de, 1f32a