Routines from the Classic app have been replaced by a combination of scenes and automations. So here’s a quick rundown of what makes them run.
1. Widgets
In the Classic app, you could show Routines as widgets. In the new V3 app, it’s Scenes. Up to 8 of them.
To select which scenes have widgets,
tap on the three horizontal line icon in the upper left.
Tap on the gear icon
Tap on “Widget” to access widget settings”
Now select/deselect scenes
Add widgets per usual for your phone/tablet
2. Watches
At the time of this writing, there was no Apple watch app and only some Samsung models had one. Instead, try IFTTT. (Note however that as of October 2020, you can only have three free applets with Ifttt. Otherwise you will have to pay a monthly fee.)
https://support.smartthings.com/hc/en-us/articles/211978466-IFTTT
(Note: there is no longer a separate IFTTT “do“ app, the functionality has been folded into the “Button“ service in the main Ifttt app, which is available for phone, tablet, or watch.)
There is also a new third-party app for the Apple Watch called JSWATCH which has gotten good reviews in the community. Discussion thread here:
3. Run a Scene Manually
Once you have favorited some scenes, they will appear on the main screen of the app and you can just tap them to run them. So how do you get them to appear there? You have to make them “favorites.“
Tap the 3 horizontal line icon in the upper left
Select “Scenes”
Tap the three vertical dots in the upper right to get to the edit screen
Now you can select or deselect your favorites
.
You can also change the sort order, but the only choice is alphabetically or by creation date. 
4. Run a Scene by Voice
Scenes, whether favorited or not, will be automatically discovered by your voice assistant with your devices. (At the time of this writing, it was all your scenes from all your locations, you cannot exclude any. )
In the Alexa app, these will be listed in their own category under “Devices.“
However, both Google and Alexa have their own rules on what can be included in a scene for voice control, and they typically exclude “security” Devices including locks, cameras, and garage doors.
https://support.smartthings.com/hc/en-us/articles/210204906-Amazon-Alexa-SmartThings-Scenes
If you have a scene set up in SmartThings but it doesn’t show up in your voice assistant, check the devices being used. At the present time if it has anything other than lights, switches, and thermostats it probably will be rejected.
To run a scene Which was rejected by a voice assistant, include that scene in an automation and use the instructions in 10) below for running an automation by voice.
5. Run a Scene Automatically
Scenes themselves just represent a moment in time, they don’t have triggers. But you can include a scene in an automation, and thereby run it automatically based on time of day, presence, mode, Or anything else which can trigger an automation.
Select the + in the upper right
Choose Add Automation
Set your “If” conditions
Choose “Run Scenes” for the “Then”
6. Run a Scene from IFTTT
Include the scene in an automation and trigger the automation with a simulated switch. Then use Ifttt to turn the simulated switch on. You will also need to turn the switch off again so it’s available to use the next time. You can use the “power allowance“ feature in smart lighting for this, or have the automation turn it off.
8. Turn a Scene Off
You can’t. That’s not how scenes are designed. (This is true of most home automation systems.) A Scene is just a moment in time. So instead, you need to switch to a different scene with different device states.
9. Run an Automation Manually
Automations are if/then statements. To run one manually, include a simulated switch In the “If” section. Then when you turn that switch on, if all of the other Conditions in the if section are also true, the automation will run.
You can turn the switch on by toggling the switch itself or by putting the switch into a scene and running the scene.
In this screenshot, the circled scene just turns on a simulated switch. That simulated switch coming on then triggers an automation because the simulated switch is listed in the If section of that automation.
You will need something to turn the switch off again so it will be ready for the next time. There are several different ways to do this. You can turn the switch off in the automation itself. You can use the power allowance feature in smartlights. You only need the trigger switch to come on for a few seconds in order to start the automation, and then it can turn off again right away.
10. Run an Automation by Voice
Use The method in 9. above and add a simulated switch to the “if“ section of the automation.
You can then either just turn on the simulated switch by voice, or you can put the switch into a scene.
Some people prefer to use scenes with voice assistants because then they can say “Run Bedtime” or “Activate Bedtime” or “Bedtime Scene” rather than saying “turn on bedtime.“ And they just find that more natural. Also, a scene can be a widget. But there’s no other difference between just using a switch or using a scene that turns on the switch.
11. Run an Automation Automatically
All automations run automatically when their if statements are true. In the new app you have the option of either saying all of the If statements have to be true (“AND”) or any of the If statements have to be true (“OR”). It’s up to you.
12. Run an Automation from IFTTT
To run an automation automatically from IFTTT, use the simulated switch method from 9 above. Then just use Ifttt to turn the switch on.
14. Turn Off an Automation
You can’t turn off an automation, but you can disable it so it won’t run the next time even when all of the if statements are true.
Tap on the three horizontal line icon at the top left
Choose “Automations”
Each automation has an enable/disable toggle
15. To View/Edit/Delete Your Scenes, Automations, or SmartLighting Rules
Tap On the three horizontal line icon at the top left
Choose Automations, Scenes, or SmartApps (for SmartLighting)
You will see a list of what you have created for that category.
Tap on an individual scene/automation to edit it.
Tap on the three vertical dots in the top right to choose items to delete or sort them.
