This works in two ways: 1. they "plant trees" and/or 2. they buy && hold or destroy certificates.
Certificates are limited regulated and deflationary. E.g. the EU buys X certificates from the market every year and destroys them and grants less certs each year. Every company that emits "significant" CO2 needs to have, buy or be granted certs to do so. The setup makes those certs more expensive, so every year, there's a tradeoff: do I buy certs, or do I invest in lowering my emission.
Apps such as Fairshares allow public to buy (pieces of) such certs.
When agricultural (or similar) lands are transformed back to forests, that has a real and direct effect on the ability of the environment to absorb CO2 emission.
Obviously it needs to be done well, which often is not the case. Quite often there's no tree planted IRL, just some "promise to probably do so in some future" sold instead. And quite often the tree is planted but then abandoned (so that each 2 years everything dies off and the same plot can be re-used to "plant more trees"). But that is not the only modus operandi.
Still, does sound like a good efficiency gain for those villages.
AFAIK they only do CO2 emissions, yet. But when I spoke to them, they had more planned.
My problem with such products, is that they will reach the people who need it less: people who already care and try their best, will be able to do a little better; but people who don't care, won't install and use it, yet their impact is probably relatively much larger.