db={"energy_carrier_groups":[{"_id":1,"name":"fuels","energy_carriers":[{"_id":ObjectId("6183de1b5dd889cfcdeaa70b"),"name":"oil"},{"_id":2,"name":"gas"}]},{"_id":2,"name":"electricity","energy_carriers":[{"_id":1,"name":"green electricity"},{"_id":2,"name":"conventional electricity"}]},],"tech":[{"_id":ObjectId("6183de1b5dd889cfcdeaa711"),"name":"qux","energy_carrier":ObjectId("6183de1b5dd889cfcdeaa70b")}]}

db.tech.aggregate([{$match:{name:"qux"}},{$lookup:{from:"energy_carrier_groups",let:{energy_carrier:"$energy_carrier"},pipeline:[{$match:{$expr:{$in:["$$energy_carrier","$energy_carriers._id"]}}},{$project:{_id:0,energy_carriers:{$first:{$filter:{input:"$energy_carriers",cond:{$eq:["$$energy_carrier","$$this._id"]}}}}}}],as:"energy_carrier"}},{$addFields:{energy_carrier:{$first:"$energy_carrier.energy_carriers"}}}])