The booklet from UEFA regarding the do’s and don’ts pre and post match is quite thick.
Size and position of advertisements, banners, when teams, leave dressing room, in tunnel, what way to position on pitch for pre match rituals, who and when moves 1st to elbow bump. Team photo, toss of coin exchange of pennants etc.
Music played at certain time.
Players line up, Ref waits for signal from UEFA Official at the tunnel to give signal to start match, all matches being played at that time should all start together.
Both Club’s are well aware of the rules as there is a meeting with UEFA the day before the game, the times are all detailed in the booklet.