can you explain all the steps when combat happens, to include deathknell triggers

Combat only opens through a Cleanup: when opposing units end up at the same Battlefield, the Cleanup stages a Showdown (and a Combat), and the Turn Player picks which Staged Combat to open if there are several (Rules 323.8–323.13). From there the sequence is:

1. Combat Showdown Step (Rule 464). Attacker (the player who applied Contested) and Defender are designated; Attacker gains Focus. Any triggers generated by that designation go on the Chain (Attacker first, then other players in turn order, Defender last). Players then play cards/abilities with Action or Reaction and pass Focus until all pass — the Showdown closes. If a unit dies here (spell, on-attack trigger), its Deathknell finalizes and resolves now, before any damage.

2. Combat Damage Step (Rule 465). If both sides still have units, sum attacker and defender Might. Starting with the Attacker, each player assigns damage equal to their total among the opponent's units — you must assign full lethal to one unit before moving to the next (Tank/Backline reorder this, and some cards redefine "lethal"). Only after all assignment is done is the damage dealt, simultaneously.

3. Resolution Step — Combat Cleanup (Rules 466.1, 323). This is where combat-death Deathknells fire:

• Units with lethal damage that have Deathknell (or "when I die" abilities) trigger first, as Pending Items, noting their stats/location before they leave (Rule 323.4).
• Those units are then Killed and sent to their owners' trashes (Rule 323.5).
• All units in play heal (Combat Special Cleanup insertion 3c).
• Attackers still present are recalled if defenders remain (3d).
• Only after that do the Deathknell triggers finalize and resolve — after healing, so a damaging Deathknell can't finish off survivors (FAQ 10444).

4. After cleanup (Rules 466.3–466.7). Determine Combat Result — sole remaining side wins; "No Result" if both or neither have units (which can re-stage Combat). Then establish control, clear Contested, Conquer if you hadn't scored that Battlefield this turn, and remove Attacker/Defender designations as combat ends.

Two gotchas: Deathknell is "when I die", not "when I attack" (Rule 808.1.c), and if the kill is replaced by something like a recall, the Deathknell trigger is removed from the Chain (Rule 808.1.d.1).