craft shield, craft a banner, pop em together into a crafting table and done. to get a custom cape, many use optfine. you have to donate around $10 to get a cape

To be concise. Any banner that you make can be added directly to a shield in a crafting table/your crafting menu. As for the cape, in java edition you either had to have attended any minecon in person, been a realms creator, or a few other random reasons that mojang gave out capes for. However, there are mods such as optifine, that make it so everyone with optifine can see your cape (as long as you pay 10$ for that cape)

so make a banner, in a craftinf table add it.
and cape, only optifine.

Connect shield and banner in crafting table

Without 3rd party modifications like optifine or forge mods, you cannot add custom capes.
Also can you send a vid where they have custom shields, because banners are the only documented way of shields having patterns