Michael Saylor's company, MicroStrategy, hints at buying more Bitcoin, signaling a new buying spree.MicroStrategy, with 528,185 BTC, remains the largest public Bitcoin holder, followed by Marathon Digital with 47,531 BTC.US-based ETFs dominate institutional Bitcoin holdings, with BlackRock's iShares Bitcoin Trust leading at 570,983 BTC.All Bitcoin ETFs collectively hold 1.29 million BTC out of the total supply of 21 million.