Mostly just patience at this point.

They are still small enough that you can transplant the ones that are growing 2 seedlings into a new cup. Growing 2 plants that close to each other will result in 2 stunted plants. You want them 1 plant per pot for best growth.

You will also need a larger cup soon. If you see the roots hit the bottom of your bottle, it's time for a bigger cup/pot. Trimmed bottles are not ideal because the edges are sharp and can damage plants in the wind. 16 oz solo party cups work well.

Here is one of Kangstarr's videos talking about seedlings, that are about the age yours are at. He covers fertilization of young plants in this vid too.

https://www.youtube.com/watch?v=ML1tPC3wLVU He has tons of other helpful pepper videos on his channel. I learned a bunch from binge watching his vids.

You can feed them VERY sparingly at first, one of the classic noob mistakes is burning the seedling with nutrients trying to get them to "grow faster".

You plants are looking pretty good... Just takes time, peppers are not the fastest growing plant by a long shot.

Here is a link to the fertiliser pellets he uses https://www.amazon.com/Osmocote-Vegetable-Smart-Release-1-Pound-Fertilizer/dp/B00G8FF2UO You want the one with the green label that says "Flower and Vegetable" it has a balanced NPK of 14-14-14