Combining two results in cypher

I am trying to combine the result of two MATCH statements.
for example like this

MATCH (a)-[:connection]-(b:labelB)
WHERE a.id IN $selection
WITH COLLECT(a) + COLLECT(b) AS selection

However, the issue with this is that it produces a list of type List<node> instead of node. This is an issue because it does not enable another MATCH statement using selection.
Meaning this would not be possible as a continuation to the query:

MATCH (selection)-[]-(c)
RETURN c

This behavior can be achieved by a UNION but it seems to behave unexpectedly when using the result of the first MATCH in the second MATCH.

MATCH (a)
WHERE a.id IN $selection
RETURN a AS selection
UNION
MATCH (a)-[]-(b)
RETURN b AS selection

How can I combine two Nodes into a single value?

You can use UNWIND which is similar to a WHILE looping in sql.

MATCH (a)-[:connection]-(b:labelB)
WHERE a.id IN $selection
WITH COLLECT(a) + COLLECT(b) AS selections
UNWIND selections as selection
MATCH (selection)-[]-(c)
RETURN c

You could also just replace this:

MATCH (selection)-[]-(c)

with this:

MATCH (s)--(c)
WHERE s IN selection

By the way, I should point out that your first query will cause selection to have duplicate nodes if it is possible for a connection to connect 2 labelB nodes.