Tcould learn dark magical secrets and leech power off their patron.
The flavor is similar in that you're an unholy wielder of demonic power (if you choose Fiend as your pact) but different in that WoW warlocks become the evil master while DnD warlocks become a willing servant to boost themselves.
Warlocks doesn't draw their power from their patrons. They don't "leech power". They only learn from their patrons and have his soul "infused" by eldritch energy. This since 2e in complete wizard handbook(where the class appears as witch). On 5e, during playtests, WLK was INT based and only changed to CHA based due player feedback.
As for summons, warlocks on 5e are very limited in what they can summon
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But one thing. Your char on BG3 is at best lv 4. Ultra low level adventurers can't cast mid or high level spells to summon powerful demons.
For eg, summon greater demon requires 4th tier magic. Planar Binding a 5th.
The summon greater demon spell description is here >
https://www.dnd-spells.com/spell/summon-greater-demon