The ending of Venom: Let There Be Carnage is that Venom consumes Cletus and Carnage, Frances dies when the cathedral collapses, and Eddie and Venom reconcile as fugitives.
The cathedral fight is decided by a trick rather than raw strength. Venom is nearly beaten, so he provokes Frances into screaming, and her sonic power is the one thing that hurts Carnage. That separates Carnage from Cletus, and Venom can finally eat both. In my opinion, this is the smartest beat in the film, because it was planted in the 1996 prologue, where a young Frances is taken away for the very scream that later brings down her husband to be. The villains' bond is the weakness, not their power.
The film also earns its reconciliation through structure. Venom walks out on Eddie after their bitter argument, hops from host to host around San Francisco, and is found by Anne in the company of Mrs. Chen. Eddie meanwhile is arrested on Mulligan's suspicion, so each half of the pair spends the middle of the film without the other. Perhaps that is why the reunion lands: Cletus and Frances arrive at the cathedral as a matched couple, and Eddie and Venom arrive as the same odd couple they were at the start, only now sure of each other.
Production context supports that reading. Syfy reported Serkis saying that Tom Hardy worked on the story with returning screenwriter Kelly Marcel, and that the film centered on their take. At 97 minutes it is shorter than venom 2018 at 112 minutes and Venom: The Last Dance at 109 minutes, so the squabbling pair gets less room than in either film, and the plot moves fast.
Mulligan survives with a piece of Carnage in him, which leaves a loose end. It could be argued that this is the kind of dangling thread a sequel would pick up, though the third film moves on to Knull and a military hunt instead. Eddie and Venom are now wanted by the law, but the argument at the center of the film is settled, and they take the beach holiday Venom wanted. The mid credits scene then pulls them toward the spiderman far from home era, where J. Jonah Jameson is already ranting about Spider-Man on television.
Is Carnage Venom's son? The film shows Carnage growing from a trace of Venom that Cletus swallowed, so it is an offshoot of Venom rather than a son in a family sense.