Yes, preseason matches sometimes don't provide a clear picture of a team's true performance. Last season, I often saw teams perform poorly in preseason but then perform well when the season started. And vice versa. So, we can't even judge a team's overall ability based on preseason alone.
However, for teams with several new players, the coach will usually test them by placing them in different positions, where they will then assess their suitability. And sometimes, new players don't perform as well as expected, as is the case with Gyokeres at Arsenal right now. But again, this is just preseason, and what we'll truly see is when the season actually starts.
Yes, coaches will test everything in the preseason, including new players acquired through the transfer market. Even the repositioning of existing players is sometimes tested in the preseason. Because live matches make it easier to assess each player's abilities. Therefore, we can say that preseason matches sometimes serve as a trial for each player, as the coach will assess each player individually in the squad. As a result, we always see all players alternately included in preseason matches, and it's rare for a player to be left out. Even reserve players will get a chance to play, because that's where the coach gathers direct data on each player's abilities and potential for the new season. Strategies and formations are also sometimes tested in the preseason. But we know that sometimes the formations that will actually be implemented in the actual season are rarely shown in the preseason.