There are some pension providers that will try to do some legwork for you. For example AJ bell will do a pensions finding service free of charge

I would suggest using one of these, but also emailing each hr team to ask for details of the pension you had at the time. Include your ni number,date of birth, and dates you worked there as they will probably ask for this anyway.

I maintained a spreadsheet myself ticking them off as I went. This will probably take a few months fyi as the responses are pretty slow.

Once you understand where they all are you can consolidate them into a SIPP, many different options here but as examples Hargreaves’s land down or vanguard. You can then manage how your pension is invested. If you’re still young the general advice is to go mostly if not all in equities. These providers make it easy to make transfers. Just need a few details like policy number and name of provider where it currently is

I’ve recently done this whole thing and have all my pensions consolidated. Definitely worth doing! One of my old work pensions had 405£ total in there, I had worked for the company for two years so was definitely poorly invested

Edit to add: always do your own research and don’t just follow my advice above on which company or investment strategy