I checked Enable Pre-release packages under Edit → Project Settings → Package Manager and restartet Unity. But I can’t find any in the Package Manager.
Is there anything else I have to do to e.g. find the Jobs package?
I’m using an asset which in its docs says it depends on Burst, Mathematics, Jobs and Collections. I do see Jobs in the preview packages listed: https://docs.unity3d.com/Manual/com.unity.jobs.html
I don’t find any package named Collections though. Not even in Experimental state. Was that package removed?
In theory you don’t need the Jobs package because that just adds additional Job options for ECS. But if the asset was written this way, oh well. As for the Collections: you can always add packages by name. Hit the big + button and choose the Add package by name option. Use the com.unity.collections name.