A program says a DLL is missing

The answer is almost never to find that file. Each of these belongs to a runtime package, and installing the package puts the DLL where it belongs, registered properly. Below is which package contains what, with the publisher's own download and its SHA-256.

Microsoft Visual C++ 2015-2022 Redistributable

• vcruntime140.dllGames, media software and anything compiled with a modern Visual Studio. One of the most common startup errors on Windows.
• msvcp140.dllThe same programs that report vcruntime140.dll. Often both are missing together.
• concrt140.dllApplications that do heavy multi-threaded processing, typically media encoders and games.
• mfc140u.dllOlder-style desktop applications with classic Windows dialogs and toolbars.
• api-ms-win-crt-runtime-l1-1-0.dllInstalling modern software on a Windows install that is behind on updates.
• mfc140.dllDesktop applications with classic Windows dialogs, toolbars and property sheets.

Microsoft Visual C++ 2013 Redistributable

• msvcr120.dllSoftware released roughly between 2013 and 2016, and games of that era.
• msvcp120.dllThe same era of software, usually alongside a msvcr120.dll error.

DirectX End-User Runtime

• d3dx9_43.dllGames from roughly 2005 to 2015. Easily the most common DirectX error there is.
• d3dx9_39.dllGames from the late 2000s.
• xinput1_3.dllGames with gamepad support released before roughly 2014.
• x3daudio1_7.dllGames with 3D positional sound from the DirectX 9 and 10 era.
• d3dcompiler_47.dllGames, and browsers or Electron applications that use hardware acceleration.
• d3dx9_42.dllGames from the late 2000s and early 2010s.
• xapofx1_5.dllGames from the DirectX 9 and 10 era with positional or processed audio.

NVIDIA PhysX System Software

• physxloader.dllGames using PhysX for cloth, debris and fluid effects.

Microsoft Visual C++ 2010 Redistributable

• msvcr100.dllSoftware and games from around 2010 to 2013.
• msvcr100.dllSoftware and games released roughly between 2010 and 2013.
• msvcp100.dllThe same era of software, usually reported alongside msvcr100.dll.

Microsoft Visual C++ 2012 Redistributable

• msvcr110.dllSoftware from roughly 2012 to 2014.
• msvcp110.dllSoftware from roughly 2012 to 2014, usually alongside msvcr110.dll.

Supplied with the game by Steam

• steam_api.dllA game that fails to start after an interrupted update or a file that got quarantined.

Windows itself, not a download

• d3d9.dllOlder games, and very often after installing a graphics mod or reshade wrapper that placed its own d3d9.dll next to the game and then broke.
• kernel32.dllStartup failures, "entry point not found" errors, and old software written for a much older Windows version.
• ntdll.dllApplication crashes reported as "faulting module ntdll.dll", which is one of the least specific error messages Windows produces.
• dsound.dllOld games, and very commonly after installing an audio wrapper or mod that dropped its own dsound.dll into the game folder.
• xinput1_4.dllGames with gamepad support, usually on a Windows installation with damaged system files.

Media Feature Pack for Windows N editions

• mfplat.dllVideo playback failures on Windows N editions, which are sold in Europe and Korea without bundled media components.

Your graphics driver

• opencl.dllVideo editors, rendering software and anything doing GPU-accelerated computation.

Bundled with the installer that failed

• unarc.dllInstallation failures reporting "an error occurred while unpacking: archive corrupted", almost always with a large game installer.

Supplied with the game by its publisher

• unityplayer.dllA Unity game failing to start after an interrupted update, or after antivirus quarantined a file.