For Catholics a Door to a Church is considered Holy since it is a gateway from the secular profane world with the reality of sin to a Church where the Sacraments, visible signs of God’s Grace are fully made available by Christ to the faithful (his Body the Church]. So a Church is a Holy place, a Door to a Church by extension is Holy, the altar in the center of the Church is Holy, the Bible read in Church is Holy, the Icons of and paintings of Christ and the Saints are considered Holy.
The are sacred because they are signs of something deeper in that they point to the Triune God and Mysteries of the faith. We are not iconoclasts.
Technically, “holy” means “without sin,” “pure, morally blameless,” or “set apart” A physical item cannot be sinful or sin-free, as it has no soul and is not human.
As for what is a “church,” that is clearly the Body of Believers in Christ who are meeting. Romans 16:5 says this. The building is not “holy.”
In 1 Corinthians 3:16-17, we learn we are the temple of God.
Are there any insights you can provide to further clarify?