How do you write background characters in a way that makes them memorable?

I write a lot of fanfic and one of my fics (for Voltron) currently has a lot of background characters. This may be a bit long by the way. Here's some ways I've personally made them memorable:

Details:

Make each character have specific details that make them easy to tell apart from others. Maybe they have a prosthesis, an unique scar, hair color is different. Or maybe it’s how they walk or talk. Maybe they have a notable disease/disorder. One of my background characters named Ian has a prosthetic left arm, straw blonde hair, drives a lime green pickup, has a scar on his left brow, and a southern drawl. Another girl named Courtney has very noticable Down Syndrome or heavy autism (haven’t decided yet). If you go this route, just try not to put too much stereotypes into it. People are not stereotypes.

2. Give them personality:

Always useful to have them interact with your main characters somehow to shed more light on their character, even if you barely write the background character in. It’ll make it feel that much more genuine. As stated above, Ian has a southern drawl similar but different from the main character’s (Keith) own dad (will get more on the next point). I write him talking different from most of my characters. Instead of ‘You,’ I put ‘Ya’ and so on. Another character of mine has a more cajun New Orleans accent and is a tough independent woman in her 70s. Another yet is a head doctor who is named Jackson but prefers to go by Jake and is kind and doesn’t like to be like the more stiff stereotypical doctors.

3. Have the character (s) have some relation to the main characters:

Ian in my story is the main character’s (Keith) dad’s best friend. He’s technically Keith’s uncle although they have no blood relation and he provides more background into Keith’s backstory as well as Keith’s dad’s own like how they met as firefighters or what little he knows about Keith’s mom. The woman before is Kathy who was the mom of a woman who fostered Keith years back so that is how they know each other. Jake is in charge of overseeing Keith’s health while his actual doctor, Teresa, does so since she’s a specialized in kids and teens.

4. If there is no relation with the main character (s), have the character have a relation to some other character:

In another Voltron fic, I had the bad guy (yes, another character I created) named Dr Avery Stillwell who kidnaps Keith and does experiments on him. Another background character is his son, Chase Ortiz, who spies in the space program Keith was in for his dad before he realizes how crazy the guy actually is. We don’t see him much but he’s memorable for that role as he plays double agent to not tip his dad off as well as his heterochromia (aka details again). Stillwell has another spy in the Garrison as well and his relation is that he believes heavily in what Stillwell does (aliens being real and harmful to humans) hence why he’s spying and willing to assassinate Keith (supposed alien as well as witness).

5. Backstory:

Give your background characters backstories. Ian lost his arm from a hit and run crash and got his scar (which mirrors Keith’s dad’s own eyebrow scar) from being dumb on the job. Kathy was a drug addict who spiralled when her daughter died from cancer (and why Keith went back into the foster system) and just recently got clean and became a staff member at the Garrison where Sam (Keith’s current foster dad) saw her talent with dogs and got her into the canine program (aka relations to other characters). The backstories don’t have to be elabrate (I just tend to do that) but a bit of backstory will add character to them. Like Chase who lived with his mom who divorced Stillwell over his alien theories and erratic behavior and how he got in touch with his dad and believes what the man says. Not much story there but shows us that he’s a good guy who values family but will stand up for what is right.

6. Make the background characters show up randomly at times, often as a foreshadowing:

Chase was seen twice by the main characters who were sneaking around in my first fic when he was patrolling the halls late at night and talking to his dad. He didn’t get a name until fic2 of the series but he sticks out for that reason. Another character of mine in the same series works where Keith is being held which later leads to her giving the rescue team her card to get into Keith’s cell and an apology to give to Keith.

7. Have other characters talk about your background character:

In that fic, Keith’s friends (and rescue team) end up talking about both discovering Chase and where Stillwell could be hiding. We don’t see them but this info can reveal a lot on the background character. In my other fic, there’s more people I plan to bring in because Keith talked about them. Courtney is one such person because Keith saved her from her abusive bio dad (getting nasty wounds in the process) and doesn’t know what happened after that besides she survived. I want to write a scene where they meet her and found out she has been adopted by a really caring family (who happens to be related to another main character later aka relations).

That got long…that’s what I could think of off the top of my head. Hope it helps you with making your characters memorable. I myself get really into all the points above because that’s just how I am. You might add far less but all these will add to that characters so readers and you can remember them better.