In Lineage 18.1, just pressing the left Alt key seems to work, unless (this is just a theory) the app takes over the keyboard assignments.
So, for example, Google Messages (sms), Aquamail, Facebork Messenger (don't judge me, Mrs. Hook insists on using it) and a text editor,Markor, will all give me an emoji pallatte when I press left Alt. However, Softmaker Textmaker, which hasit's own key assignment shortcuts, does not.
It may depend on what soft keyboard you are using. I use the AOSP keyboard and it has an actual setting to have emoji appear when using the physical keyboard.