This post reminds us of what level you're really at. Not understanding how to defend.
And just to be clear, McTominay let the defense down on the first goal. Zabaleta would be offside but McTominay plays him onside and then lets his man get to the cross and score. Originally its Young's man but its McTominay's area and he has time to see Young isnt going with him. He doesnt have the speed nor does he sense the danger and we're 1-0
2nd no chance to do anything.
3rd Matic is ambling about in no man's land. Doesn't get in a good position to defend the pass or cut it out and neither does he pressure the player on the ball, allowing him time and space to make an easy pass. Meanwhile Shaw has a man running in behind him, so Smalling does what he has to do every game and comes across to cover the area. In any competent defense the other CB will come towards him too, making sure there isnt a big gap between them. Unfortunately we have a CM playing at CB and McTominay doesn't sense danger so, like with the first goal, he stands around instead of moving to a good defensive position and Arnautovic receives the ball between them.
Smalling could have just said feck it, Shaw is out of position and the left winger isnt helping and just let the opponent have a free run on the outside. But that would have been bad defending in most circumstances. In any organized team that would be the wrong move. But because we have a CM who doesnt know how to play CB playing next to him, perhaps it ends up being the wrong move to do the right thing. Because in actuality Smalling has to play both CB positions himself babysitting his partner. So its harsh to blame Smalling for doing what is normally the right thing to do.