Solo events - the 22-hr events that come around every day (Bottom tab on the right). The greater and lesser Solo crystals are very good sources of gems. You can also get free units from them.
As a beginning player, I suggest maximising your use of these quests:
'Save up' low-cost maps (1 Energy per step) to explore on 'Hero Use' days. It's just free stuff!
'Save up' all the Iso8 and crystals you can; and only rank up champions on 'Level Up' days. Again, free stuff!
Be sure to fight in the arena on 'Arena Wins days. You should grind as a beginner anyway - best way to get newer 3* and 4* champions.
Ignore Event Quest Completion days. Far too energy-intensive!
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As a beginning player, I suggest maximising your use of these quests: