[1] Peri Megadim 271 A”A 7; See Piskeiy Teshuvos 271 footnote 184

[2] The reason: As one of the reasons behind covering the bread is to show that the food was brought to the table for the sake of Shabbos [i.e. Yikra Deashabbasa]. Initially the custom was not to bring the food at all to the table prior to Kiddush due to this [see Admur 271/17; Tur 271; Tosafus Pesachim 100b; Sheilasos Yisro 54; Rashbam Pesachim ibid], and now that we bring it to the table, at the very least one must cover it, and there is no difference between the Challah and the other foods. [P”M ibid]

[3] So is clear from the concluding wording of Admur ibid that “The custom is to bring it initially before Kiddush and cover the bread until after Kiddush”; See Hearos Ubiurim Tzemach Tzedek 7/257

The reason: Perhaps the reason for this is because the main aspect of the meal is the bread, and only by it is applicable the concept of Yikra Deshabbaso. Alternatively, perhaps it is a Tircha to cover all the foods and hence it is not done.

[4] Piskeiy Teshuvos ibid

[5] Admur 271/17“The custom is to bring it initially before Kiddush and cover the bread until after Kiddush”; Aruch Hashulchan 271/22; Minchas Yitzchak 3/13-10; Piskeiy Teshuvos 271 footnote 184

Other customs: Some are accustomed not to place the Challas on the table until after Kiddush. [Maaseh Rav 118]

[6] The reason: In order to diminish in making an interval between Kiddush and the meal. [See Admur ibid] Alternatively this should be done in memory of the Mun. [Aruch Hashulchan ibid; mentioned in Admur ibid]