Several things here are obvious:
1. Celtic are so far above the standard of the rest of Scottish football it's laughable.
2. By virtue of winning the league every year and jumping to the 2nd Qualifying Round of the CL every season, their club coefficient is reasonably high. They stay in the top 50 by virtue of beating teams from Lithuania in the qualifiers, then flailing miserably at the group stages.
3. Their overall quality is remarkable for its inconsistency. Somehow holding Man City to a 3-3 home draw after losing to Lincoln Red Imps from Gibraltar is bizarre.
They'd likely receive a club record fee for Tierney, which would only have to be ~£15M or so to exceed what they got for Van Dijk. For Celtic fans to suggest it will take anything like a world record fee for a defender is delusional, even given the current markets. Records for fullbacks are around £50M (Mendy and Walker), and I wouldn't pay more than half of that. For £22M, the club could activate the release clause for Jķnior Firpo from Real Betis, who is the same age and looked impressive after making his debut in a much tougher league.