I can compare auction insights periods, but I cannot tell you how much a competitor spends. The report does not show that, and I will not guess it.
To find what changed, I need: 1. Two periods of auction insights rows (required): the same campaign, two date ranges of equal length, with each display domain and every metric column as exported (impression share, overlap rate, position above rate, top of page rate, outranking share). You gave one pair of numbers (35% and "they were at 20%"), but I cannot tell which is yours, which is theirs, or which period each is from. 2. The campaign name (required), so both periods are from the same auctions. 3. Changes you made between the periods (optional), such as budget or bids. Default: none known.
Why these: each metric has a narrow meaning in Google's definitions, for example "Overlap rate is how often" another advertiser's ad got an impression when yours did. A rise in their share does not by itself mean they took your clicks. I will show the changes in percentage points and turn possible reasons into questions.