When do you need to file a self-assessment?

So...Ive spent years now trying to wrap my head around US taxes abroad and I now find myself confused about the other way.

When I moved to the UK years ago, I asked around about taxes. I was told that if I had no income sources other than PAYE, that I did not have to file a self-assessment. And when I looked into opening a basic interest yielding (non-ISA) savings account here, a google search told me that I did not have to do a self assessment unless I earned over 1k in interest. (I do still report this interest to the IRS.)

Well, my UK account doesnt earn much interest and my only income revenue is my employment. I recently started looking into the conplexity of investing as a dual citizen living in the UK because i have FINALLY paid off my american student loans and now actually have an expendable income. In looking into this I have come across a question that has me panicked - does the UK require disclosure of all foreign accounts like the IRS does?

When I moved here, I shut down most things except an interest-free checking account that Ive had with my mom since I was a teen purely so that I could send money home in order to pay my student loans. (The companies would not accept payments in foreign currencies.) This account is still open with a meager minimum balance present and sat there doing nothing. I also use it to send home money to pay my yearly life insurance premium.

Was I supposed to be reporting this basic account via self assessment this whole time? I literally came over here with no money and the little money I had left after visa fees and plane tickets sat in that account at home for my monthly loan payments. I have never had investments or property of any kind in my life or high interest savings other than CDs, which I hadnt had one in years before coming here. I never even had a job where I could start any kind of retirement savings. So the idea of getting a UK tax advisor never even crossed my mind because what did I have to advise on? But now Im worried that the UK is just as complicated even though Ive lived my whole life here basically only with enough money to cover one minor emergency. And that always went down to near nothing with each visa application or the few times Ive been able to fly home.
Was so hoping now that I have citizenship and no more loans that Id finally be free to build a life but now am scared of consequences for things I may not have even realised I should have asked

I think I can safely say that you don't have to report that US bank account you mention in your post to HMRC.

My son is in a similar position in that he is a dual UK/US citizen working in the UK with a small amount of interest  (<£500) from UK savings accounts so he doesn't do an HMRC self assessment, pays his UK taxes through PAYE. He also maintains a US bank checking account which pays no interest so has no foreign income to report to HMRC. He does file a US tax return each year, excluding his UK salary through FEIE (Foreign Earned Income Exclusion), and reporting the small amount of interest earned on his UK savings accounts. He does meet the FBAR limit so also files a FBAR report each year.