Mainoo is not a defensive midfielder or a deep lying playmaker or a 6 - he still needs to develop better defensive acumen, stamina, and long range passing. He's looked his most vulnerable when primarily tasked with screening or dynamically cutting out passing lanes. His passing skills are limited when trying to connect defense with midfield or attack. ETH tried to use him in that role at times this season - and it's clear that's a poor fit.
He can play as a box to box if needed. While he lacks in stamina for a full 90 (something that'll get better with age), he is good at dribbling and carrying the ball forward through central midfield, with the ability to find short/medium passes to bring other progressive players (forwards, wingers, overlapping full backs) into play. He is also decent at tackling and finding interceptions when out of possession. But that's not his strength, either.
He's not an out and out attacking midfier/10, either. While he can dribble, he's not someone who has the strength, guile, or pace to take on the last defender consistently. While he can pass, you can't expect him to find KDB/Bruno-esque killer balls. While he can score (and has put in some worldies this season), he doesn't have that knack of turning up at the right place at the right time, yet.
The best way to use him is to have him connect defensive midfield to attack. He's press resistent, very spatially aware, agile, and gifted with clean technique. Which, as others have pointed out in this thread, means that unlike other players who try to move into space and out of pressure to receive the ball, Mainoo drifts into tight spaces between opposition lines because his skill is in transition - to receive it under pressure, dribble through tight spaces with quick feet and sudden turns, find angles for clean passes, and provide movement for quick combination plays. You instruct your CBs/DMs/FBs to keep finding Mainoo, and he'll very quickly transition play into a situation where your 10s, wingers, or forwards have the ball and are running at the last man or the keeper. It is a sort of a hybrid 8/10 role. He won't get top goals, or top assists. Don't expect him to. What you should build the team for is to help him maximize the most number of pre-assists.
At club level, he lacks defensive screening and movment/technique around him (bar Bruno) to help him succeed. Funnily enough, he's theoretically he's got the right players in the England side (Rice as DM, Bellingham driving forward, movement from Foden/Saka, ability on the ball around him), but the team is clueless about roles and instructions under Southgate, and his England teammates aren't passing to him when he's marked (out of natural habit), or providing movement for him when he has the ball (poor tactical instructions and setup, lack of faith and familiarity).
A top class manager can build an elite international side with this England squad, with Mainoo having as key a role as Bellingham, Rice, or Kane. At United, Casemiro is in decline as a defensive force, so we need a DM alongside Mainoo and Bruno. Also, the only two defenders who could consistently find Mainoo were Licha and Dalot. We need Shaw back, and a ball playing CB to either partner Licha or replace him when he's injured.