Just purchased a Broadlink 4 mini for a few devices that use IR, oddly Broadlink does not surface devices within Alexa as devices, if you create a device within the Broadlink app it is created as a remote, not a device, which kind of makes sense but does not immediately help with controlling devices either within ST or Alexa
But… as is usual there are more ways than one to skin a rabbit
To make Broadlink devices/controls/remotes useable as a smart home device in Alexa, create a basic on/off scene within the Broadlink app, once saved this will immediately appear within Alexa, assuming the user has managed to connect the Broadlink app to Alexa, again done within the BL app, once a scene is created you can then use the scene created within BL app and auto found by Alexa as an action in Alexa
Example
When this happens > voice > fireplace on.
Add action > smarthome > control scene > select your BL created scene
So that makes IR devices voice or automation accessible within alexa
To gain Smartthings control the user needs to have created an Alexa virtual switch/contact sensor within Smartthings or the IDE, Alexa will then see this device and as above it can then be selected within Alexa as a (when this happens)
Virtual switch on/off
BL scene on/off
Clunky but do able, always reliable, not 100% as Smartthings does throw issues with Alexa integration from time to time but I do not count IR integration as reliable in any shape or form, if something stops the IR signal things get out of sync, on/off buttons also get out of sync so although control is possible full and reliable control from other platforms will never be ideal
That is why my post above says an ST hub will eventually be useful to you, things need to be understood and learned, there is very little available at present that is as good as ST, Hubitat is without doubt usefull but it also has drawbacks, and for the everyday user those drawbacks will be the deciding factor between the 2
I have used ST for 4 or 5 years, with the exception of odd devices going wonky for short terms reliability has been 99%, the new app stinks but hey, what can you do