Activating more type accents on macOS

Excuse a diacritic illiterate here. From the Wikipedia page of Diacritic, I landed upon Breve , Ğ and Cedilla, Ş on Wikipedia.

Unicode Values

• Ğ U+011E and ğ U+011F
• Ş U+015E and ş U+015F

How to add Unicode Input on Mac

Go to System Preferences → Keyboard → Input sources and use the + for adding Unicode Hex Input having the symbol U+. Since you're here, check the box, Show Input menu in menu bar.

How to type the characters

If your Input source is not U+, use ctrl+space for changing the same. Holding ctrl after lifting space bar shows the list of all added keyboards.

Given the Unicode values, hold alt/option and type the characters after U+ one by one, sequentially. For e.g. hold alt, press 0, 1, 1, E. This types latin G with breve.

Unicode Keyboard is not so great, for me it stops me from jumping, deleting word by word. So ctrl+space for reverting.

Find things like arrows, boxes or currencies on https://en.wikipedia.org/wiki/Unicode#Standardized_subsets

Or Character Viewer

Use ctrl+cmd+space to summon the Character Viewer. It is also accessible in the Edit menu → Emoji and Symbols in most apps. Leave the cursor where you want to type the text and type in the search bar of the popup, "Breve" or "Cedilla". You'll see some characters that support this. You can also add them to Favourites

Or Text replacement

You can set up a replacement in Keyboard Preferences → Text for S being replaced with Ş. Then in supported apps or utilities, like Safari, or spotlight, you'll see this:

Thus I feel if you type Ş more, this is the easiest. Similar for other characters.

The easiest way to type Turkish is to use one of the Turkish input sources supplied by Apple. Make sure you have the Turkish flag showing in the top right corner of the screen (just adding it to Input Sources preferences is not enough). To see which key does what, use the Keyboard Viewer.

Another way is to add the input source called ABC Extended to the list of Input Sources and then select that in the "flag" menu at the top right of the screen. You can find the codes for all the letters you need here.

You can pretty easily add your extra characters to unused keys on the US layout (or add brackets, etc to the Turkish layout) with Ukelele.

It may be possible to add characters to the Press/Hold popup menu of the US keyboard. For info on that see this earlier question.