Data file loss?

Hi everyone, I'm writing in this community because I'm clueless on what to do.
I own a Macbook Air mid 2012 that was sloppy and too old. Therefore this summer I took it to my local device repairer who suggested to install a "new version" of Mac into mine. He didn't warn me about any data loss or the need of doing a backup prior to installing this new version of Mac to "make it faster" (verbatim), so I just handed him my computer and let him do the work.
I noticed a lot of folders just disappeared. Folders containing childhood pictures. Also my "Photos" library has completely emptied. I must suppose that when installing OpenCore Legacy he's erased all my data in this pc to make it work? Although I think it's impossible to recover this data (yes, I'm dumb, I haven't done a backup because -- who would've expected?), I am still asking if there's a way to see old folders maybe hidden somewhere in this pc or if it's possible to go back to the way my pc was before OpenCore Legacy without Time Machine (which doesn't let me see anything before "Today")?

I'm sorry if I poorly explained myself, English isn't my first language and I am not an expert with computers (therefore why I handed this pc to this shop I trusted for years - I reckon i shouldn't have).