It's not forcibly; a company is a group of people that sign a special type on contract (often called memorandum of association and bye-laws but other terms exist.)

That contract can govern all manner of different circumstances including the circumstances in which the allocation of shares are transferred.

Sometimes only a motion of the board is required, sometimes additionally a motion by a general meeting with various options for requiring super majorities across and within the different classes can be required too.

Fiduciary duties often require the Board to act in the objective best interest of the members.

In my opinion it is better model to think of a company as a club (hence shareholders often being called a Member) rather than something that you own.

Acquisitions work the same as they always have. Roughly, if half the voting shareholders approve of the deal it will go through.

Edit: from the first result in Google:

> Casalena and long-term investors General Atlantic and Accel, which make up about 90% of the Squarespace’s voting shares, have agreed to vote in favor of the transaction.

So I guess the shareholders mentioned in the subheading of the linked press release own a majority.

There will be a shareholder vote, but they only need a majority to approve the sale (assuming their equity shareholder structure is "normal" and voting rights exist for common shareholders). However, the majority of any given public company tends to be owned by institutional asset managers like Blackrock and Vanguard. They owe a fiduciary duty to their customers to act in their best interests, and when a deal offers those shareholders a large margin over the market value of their holdings, it's almost impossible for them to vote against it, even if they wanted.