I'm wondering if you're a dm and have books but no subscription, can your players in your campaign for that campaign still create characters using subclasses and backgrounds from those books?

No, but you can create homebrew copies of the relevant things. Even if you don’t publish them (which you shouldn’t and I think even can’t), they’ll be shared with other players in your campaigns.

No, but:

• It doesn't need to be the DM who turns on content sharing. If someone else wants to pay for a master subscription (even for a single month and cancel it right away, just so everyone can use it for character setup) then the campaign can have content sharing.
• You can create a campaign, people can put their characters, partially-made, into the campaign, and then unassign the character. This will leave it in the campaign, the DM can assign the character to themselves, make the upgrades, and then unassign it so the player can pick it up again. The changes should stay, but it's time-intensive for the DM, and I think you'd have to do it at every level, so it's more time-intensive at lower levels where you level up faster. Depending on how many players you have and how often you meet/level up, someone paying for a master subscription (and people can trade off, too - person A does the first month, person B the second month, etc - once you pay for the subscription, even if you cancel it early, you get the number of months you paid for, it just doesn't auto-renew, that's what you're actually canceling) might be worth the value of your time spent doing it for everyone.

How do you do the homebrew on D&D beyond,  i mean how do you program it in?

Quote from Avenger_80 >>

How do you do the homebrew on D&D beyond,  i mean how do you program it in?

All the tools can be found under the Collections menu at the top left of the site. For technical tips and instructions etc I'll refer you to the relevant forum: https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules