T212 make their money on the spread in CFD and also from share lending.
Both of these allow the Invest and ISA to be free. They have mentioned they have no plans to charge for this and as @Lonford mentioned been running successfully for over a decade.
They have released their beta of the new platform they are building and have announced recently of an advanced view and that again it will be free. They have also mentioned when pre and after hours trading is added to invest/ISA that will be free too.
The only mentioned item that would be a paid option when available would be L2 and tape access, and that’s due to the significant cost for them to license. At the moment no UK broker offers free access.
T212 are essentially using IB as a platform underneath so they are using their API.
Freetrade on the other hand are doing it themselves connecting with the exchanges APIs. They haven’t built a CFD platform so need to charge customers a fee to support the business.
They plan to offer ISA, access to a wider range of stocks, and limit and stop orders for £9.99 a month. All the same stuff T212 has had for years for free.
Freetrade is adequate for the casual retail investor who doesn’t really want to get involved, they can buy some shares and peak at it now and then making sure the wiggly charts are going upwards. Instead of paying £3 you could do the same in T212 but see the actual ask not the last trade and have access to far better charts and a huge range of choice.