The ark was the throne of Yahweh where He dwelt in a localized way and met with the Israelites through their high priest. It was the seat of His sovereignty but also the place where He met with His people ( Exodus 25:22). This is why directions for its construction come first. The testimony (Ten Commandments, Exodus 25:16; Exodus 25:22) lay inside the ark, which was a hollow box. God"s dwelling among His people and His relationship with them thus quite literally rested on the Ten Commandments. The mercy seat ( Exodus 25:17) was the removable "lid" of this box and was solid gold. It was there that the high priest offered sacrificial blood once a year to atone for (cover) the sins of the Israelites as a nation. This offering made propitiation (satisfaction) for their sins for one year (cf. Leviticus 16). [source][source][source]
The Greek word used to translate "mercy seat" here in the Septuagint (hilasterion) is another form of the word used to describe Jesus Christ as our propitiation in 1 John 2:2 (hilasmos). The mercy seat was for the Israelites temporarily what Jesus Christ is for all people permanently: the place where God found satisfaction. [source][source][source]
"It [1] is a sweet word! A seat of mercy, baptised [2] in mercy, from which mercy flows forth. Not wrath, not judgment, not indignation, but mercy is pouring forth from its original fountain in the heart of God." [3][source][source]
The cherubim ( Exodus 25:18) were angels who "apparently have to do with the holiness of God as violated by sin." [4] They may have looked like winged human-headed lions. [5] Josephus wrote that Moses saw these creatures around God"s throne when he was on Mt. Sinai. [6][source]
"The cherubim are connected with the throne as its guardians and/or bearers. In other cultures cherubim are minor deities protective of palaces and temple; in Israel they symbolized angelic guardians of the invisible throne of God." [7][source]