I don't understand the question. You can buy compost and it may be a
certain kind, eg mushroom, but mostly it is just some brand name and
they don't say what is in it. OTOH if you make your own it is
whatever it is, according to what you are composting. OTOH there are
commercial potting soils which are not compost but may contain
compost. Can you clarify?
Yes there are water retention beads and wetting agents that may help
with water retention. See your local nursery.
Vermiculite and perlite are additives that can help retain water. Moat
compost sold in stores are animal manures. Plant manures may be better and
typically are free at local communities recycling centers.
Enjoy Life... Nad R (Garden in zone 5a Michigan)
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