It's not that easy to find something useful about it online, I agree. Accordingly to Timothy Arndt and Angela Guercio in Decomposition of data flow diagrams,
Data flow diagrams are an important design aid in system development.
Decomposition is the top-down development of a data flow diagram
starting with the system inputs and the system outputs
Personally I liked the content from Visual Paradigm on the matter to know more about DFD. About decomposition, they write
Top-down decomposition, also called leveling, is a technique used to
show more detail in lower-level DFDs. Leveling is done by drawing a
series of increasingly detailed diagrams until the desired degree of
detail is reached.
In other words, when doing leveling the goal is
first displaying the targeted system as a single process, and then
showing more detail until all processes are functional primitives.
When you ask in your question
some say data store and sources/sinks are not allowed in the level-1 and the rest of decomposed diagrams, but some say it can be
what i extract from that article in Visual Paradigm is that in a context diagram you don't want any data source but in a Level 1 it's fine (there's even one example of it there - see next image).
In fact, there's some considerations / criteria to have for DFD leveling: unique names for levels, no cross line, numbering convention, ...
I suggest you to have a look as well at this article from Visual Paradigm which explains DFD by using a food ordering system as an example.