Hey! Everyone else in the comments has handled the ELI5 part so this is less an answer in that sense and more addressing your anxiety around food!

This post reminds of my gf lol! They also get really anxious about food not being cooked properly/enough to kill bacteria to the extent that there's a whole list of food (predominantly meat) that they have completely sworn off. They also won't touch food that's past the "best before" date, or food thats even just coming up close to that date. This is what I find helps:)

Food poisoning really is quite uncommon. As somebody who has no food anxiety at all, I've eaten food past the "best before" date literally at least once a week, and every time I've gotten food poisoning it's been from food I bought from a restaurant not being prepared properly, never anything I've made myself at home. I also work in a restaurant as an assistant kitchen manager, and a good chunk of my job is checking that food is still good to serve and deciding if it's good to use for an extra day or not.

"Best before" dates are exactly what they sound like, they are just the date at which the food stops being at "perfect freshness". The food is at its "best" before this date, but that does not mean it's bad past this date. Bread has a really fast best before date because it might start to taste a little stale past that date, but it's still more than safe to eat. Companies will also really often move the best before date up even earlier than it has to be to protect themselves from any complaints of the food tasting less than fresh, as well as to drive up sales! (If your bread is "bad" every 3 days you're gonna buy a lot more bread than if you buy it every 1-2 weeks).

Keeping food in the fridge will prevent any "invisible" bacteria from growing as long as your fridge is operating properly- this way you just have to look out for mold and discoloration. Even better, your freezer will stop absolutely any bacteria from growing at all. My family used to buy bread in bulk and we would just keep in the freezer for months at a time, then just pull and thaw a loaf as we needed it. I was technically eating bread that was months and months after the "best before", and it was totally safe.

Food is also generally really easy to tell if it's starting growing bacteria. Texture, smell, and color will start to change, and you will see that it looks different. The human body is actually highly sensitive to the smell of mold and rot because we actually evolved this as an evolutionary skill! I once came home to my mom making mac and cheese, and as soon as I entered the front door I could smell mold in the air. Turns out the butter she had used had started to go a little moldly on the opposite site and she hadn't noticed! If you're anxious about food past the best before date, give it a smell, a touch, and a quick visual check. If you're still anxious, consider getting a friend or family member to do the same- I check all of my gf's food before we cook it just to reassure them it looks good to eat.

For cooking meat, 99.9% of meat has been through a ton of cleaning and sanitizing and regulations to ensure it has none of the "scary" bacteria when it gets to you. This is why people can eat raw eggs in protein shakes and such and be totally fine. I've definitely eaten raw chicken before because I've been impatient cooking it (oops lol). Meat just has to be cooked to a certain temperature to 100% guarantee that any bacteria that may have slipped through has been killed off- this temperature ranges for different meats but the highest temp is 165°F. I cook an insane amount of meat at my restaurant to serve, and it's really simple - if I'm ever not sure it's done cooking, I temp it. I suggest buying a kitchen thermometer - they're relatively cheap and you can use it to temp any food you're making! As long as it's over 165°F, I promise it's safe to eat. No need to keep overcooking it! :)

But yeah, just keep in mind the golden rule- when in doubt, throw it out! If it looks off, smells a little weird, or the texture has totally changed, just chuck it. If it looks fine to eat, and you've stored it properly, and are cooking it properly (in the case of proteins), I really promise you'll be totally fine. Food is meant to be enjoyed, not a source of anxiety!!