Source on Android
Source Engine On Android is a port of the 
Source Engine to Android platforms. It was based off of NVIDIA's Shield Platform port of 
Half-Life 2 and 
Portal.
Before this port was released, this was the only way of playing these games on an Android based system.
Newer Releases use the leaked Source Engine 2018 code and abandoned the Nvidia Shield Version.
History
In 2019 a user named "nillerusr" started uploading development videos to YouTube showing off a modified version of the NVIDIA Shield version of Half-Life 2 running on a standard Android device.
In his first video on the topic he shows off the touch capabilities of the mod.
Later versions of the port were based on leaked Source Engine code (specifically Team Fortress 2 version of Source 2013) dated back to 2017-2018 (which was leaked in 2020).
A public build would be released on October 30, 2020. This build included playable versions of Half-Life 2, 
Half-Life 2: Episode One, 
Half-Life 2: Episode Two and Portal.
Since then many other games have been ported such as 
Half-Life 2: Deathmatch, 
Half-Life: Source, 
Day of Defeat: Source and 
Counter-Strike: Source.
Installation
Warning:While the game files can be copied over from your PC it is highly recommended to use the download script since it is faster and avoids corruption.- Download the latest Source Engine apk and then the game apk of your choosing (the Source Engine app launches base Half-Life 2); see the External links section below.
- In the phone's storage create a folder called
srceng(or anything really) - Install Termux.
- Once inside Termux, install Depot Downloader
- Download the games using this as an example: depotdownloader -username user -password pass -app 220 -depot 221 -dir HL2 -validate -branch steam_legacy
- Verify with the Steam Mobile app.
- Go to your Downloads folder, find "depotdownload" directory and copy over the game to the srceng (or whatever you named it) directory.
- Choose parameters and launch.
Tip:If the gallery app picks up the images from the game files, place a ".nomedia" file in the srceng folder
Note:(Half-Life 2 only) This port is only compatible with pre-20th anniversary version files, these parameters have to be used -branch steam_legacy to choose the steam_legacy build for Half-Life 2.Installing Other Games
To install games besides Half-Life 2, simply copying the folder of the same name as the game to the srceng folder is all that is needed.
For example, to install Portal, these parameters can be used -app 400 -depot 401 for Portal's resources which have to then simply be copied to the srceng folder. Then Portal can be launched from the separate Portal app.
The following diagram showcases the game folder structure:
srceng
|
├── hl2 --> Half-Life 2, (REQUIRED FOR ALL GAMES)
├── platform --> Half-Life 2, (REQUIRED FOR ALL GAMES)
|
├── hl2mp --> Half-Life 2: Deathmatch
├── episodic --> Half-Life 2: Episode 1
├── ep2 --> Half-Life 2: Episode 2
|
├── hl1 --> Half-Life: Source
├── dod --> Day of Defeat: Source
├── cstrike --> Counter-Strike: Source
└── portal --> PortalNote:If help is needed, questions can be asked on the official Discord server: niIlerusr's Discord server
Installing on an SD-Card
Installing all the game files on a SD-Card can be beneficial if the internal storage is full or the user just prefers it that way.
However, since the app only allows for the selection of directories from the internal storage, the SD-Card path has to be entered manually.
Warning:Installing the game files on the SD-Card is not recommended by the developers
Getting the path to the SD-Card alone can be challenging since regular file managers just report it as SD Card and don't give the actual path to it.
The easiest and least "ad-riddled" way of getting the SD-Card path is to connect the device to a computer loaded with ADB (Android Debug Bridge)
To use ADB, USB debugging has to be enabled which varies from OS to OS.
Tip:Simply look up your device on the internet for instructions on enabling USB debuggingOnce all that has been done, a command prompt has to be opened in the folder in which ADB is located in (this can be done by typing cmd in the address bar)
Then to confirm that the device is recognized by the system, typing in adb devices will show all attached devices.
If a string of numbers and letters followed with device shows up, that means that it has been detected.
Note:A pop-up will show up on the phone asking you to authorize the PC, press yes.Next, type adb shell
Then, type df. This will list all directories on a root level. The directories we are interested in are usually at the bottom.
There should be 2 directories starting with /storage/.
The internal storage will always be /storage/emulated while the SD-Card is usually /storage/ followed by a 8 character string divided into two by a dash (-)
For example:
/dev/fuse 109783884 53449520 56203292 49% /storage/emulated <- The Internal Storage
/dev/fuse 124835840 35815296 89020544 29% /storage/9257-194E <- The SD-CardThe path specified (in this example /storage/9257-194E) is what we will write in the "Path to game resources" dialog in the app.