TapNoiseIn supporting Mac apps, Control-Command-Space opens the emoji and symbol picker. Click where you want the character first so it is inserted into the right field.
Use the picker intentionally
Open the picker, search for the emoji or symbol and choose it. Test in a scratch document if the app handles rich text, code or form fields differently. An inserted symbol is still text and may not be accepted by every field.
You can also inspect Edit › Emoji & Symbols in the app’s menu. The menu is a useful fallback if a keyboard combination has been changed or reserved elsewhere.
Check the Globe key
On keyboards with Fn or Globe, macOS Keyboard settings can assign that key to emoji, input-source switching or another supported action. Inspect the current assignment before assuming the key is broken.
Do not change it solely to match a tutorial if you rely on its existing behaviour. The menu command remains available without altering your keyboard setup.
Pick the right tool for the character
Character Viewer helps you browse symbols. Keyboard Viewer instead shows where characters live on the current layout. For frequent accented letters, the accent picker or Option sequences may be quicker.
In code, check whether the file requires plain ASCII characters. A visually similar mathematical symbol, smart quote or unusual space can be different text even when it looks correct on screen.
Sources: Apple’s shortcut reference, Keyboard settings.