Or, very occasionally they are de facto buying software support with "donations".
Say I own a small business. I can make a donation to anything i like because the only person who might complain is me. Ultimately I am donating my own money.
If I work at a large company, all the money belongs to other people. Naturally I need to spend money to get stuff done, I'm given a budget and some rules framework.
To avoid corruption, theft and embezzlement, that set of rules has balances. People who audit my spending etc.
Imagine they set a rule saying I can spend $100k on "random open source". Cool, my wife makes an Open Source flappy birds clone. 100k to her.
Not cool, it must be something we use. Cool. My wife has forked a text editor (say emacs), made a couple spelling changes, and I've mandated that as our one-true-editor. 100k to her.
You get the picture. Donations are indistinguishable from corruption.
As a company what I want is for you to -sell- me something I can justify. Not just "the same thing under a different license" which spawns the "why aren't we using the free one" conversation, but some feature or benefit I can point to. We're spending 100k, but we got the following economic benefit from that.
So to answer your question. Companies can't "give", they can only "buy". (For any significant amount.) If you want corporate money, have a product to sell, and a proper process (invoicing etc) to process sales.
Its not an easy sell.
Several people have done fine work in providing example sales pitches, articulating the benefits of supporting open source generally and maintainers specifically. Even with that kind of help, it's still a hard sell.
Many business are run for today with no regard for tomorrow, stiff suppliers and contractors and come back next quarter whining about "but if you won't extend more credit you'll be taking hundreds of jobs!" Actual legal obligations don't mean much to those people; they'll never care about a commonwealth.
The better type of business still has far more things to do with their money than care for an apparently healthy and vibrant common ecosystem. Usually, the best they can do some public relations spending on par with the rest of their charity work.
I have had discretionary budgets many many times. I could easily give part of that budget to open source maintainers but I’ve also never found one that made it easy to get an invoice.
https://news.ycombinator.com/item?id=39905557
Corporations are paying in equal measure.
To be clear, this model exists, and is adopted by lots of startups and OSS companies when they decide revenue is necessary. But at that point they are no longer OSS and typically their very customer base, who have been selected because they care about OSS defect.
A golf club attracts members who want to play golf. Tennis is more profitable so they bulldoze the golf course and build tennis courts. Don't expect the current members, who were there to play golf to hang around.
If you are starting a business I encourage you to figure out the revenue model first, not as an afterthought. Hint "getting lots of users" is not a revenue model.
There are a few more examples in addition to Target over at https://fossfunders.com. There is a spectrum of approaches, but it's encouraging to see some companies willing to give financial support without necessarily expecting support or services in return.