Both ends of the stack at moderate depth, with the integration and API-boundary questions specific to full-stack loops.
Say yes if it is true, give a stage and a rough timeline without naming companies, and use it to set a decision date rather than to imply a bidding war. Inventing a process you do not have is the one answer that can end badly.
Give the reasons in one pass so the pattern looks like a sequence of decisions rather than a habit, then name what you have changed about how you choose a role. The answer that works is a filter you can state, not a promise to stay.
Answer no, and give the non-money reason you are leaving as the evidence. If a real counteroffer arrives later, judge it against what your employer did before you threatened to leave, not against the number on the paper.
Ask for the deadline and the reason in writing, then request a specific short extension while telling the other process you have an offer with a date. A deadline with no reason behind it is a negotiating tactic, and it is also information about the employer.
Ask what the panel saw that put you at the lower level before you argue, because level drives your band and your next promotion, not just this year's pay. Then either counter with evidence against their own criteria, or get the path to re-levelling in writing.
Read the paper against what you were told and query every difference, because only the document survives a change of manager. Check how variable pay is defined, what probation and clawbacks say, and get any verbal arrangement written in before you sign.