Poppin’ relics open is a stable part throughout the game. Which can be done regardles of level.
Relics are 4 person dungeon content.
You open “in game dropped loot boxes” by running a normal mission and picking up specific rewards that drop from special spawned mobs. Where you can make the more rare reward more obtainable, pick between your own or the other player’s loot reward. Play for hours or just for 5 minutes (depending on type of mission or how many you want to queue for), etc.
Rewards and Drop Locations: https://warframe.fandom.com/wiki/Void_Relic#List_of_Void_Relics_and_drop_sites