I've deleted all instnaces and relevant services, why Free Tier storage (30 GB-Mo) is still full?

Billing and cost management the Free Tier storage (30GB) is still full. Please explain the reason

Cost Explorer can take 24 hours to update https://docs.aws.amazon.com/cost-management/latest/userguide/ce-what-is.html

The current month's data is available for viewing in about 24 hours. The rest of your data takes a few days longer. Cost Explorer refreshes your cost data at least once every 24 hours.

The second part of your question:

and guide, how can I free up the Free Tier storage for new instances?

Free Tier gives you 30GB of gp2 or gp3 EBS volumes per month. This basically means you can provision 30GB at midnght on the first of the month, and keep it for the whole month, and it won't cost you anything, assuming your account is less than 12 months old, and that you don't provision any other storage in that time (actually, the way the arithmetic works, you could also provision 60GB on 1st of the month and delete it all on 15th of the month. or provision 90GB on 1st of the month and delete it all on 10th of the month, and you should still be okay. But that's not what you're asking).

Today is the 22nd March, which is approx 70% through the month. If, as you have described, your account is in scope of Free Tier and you have provisioned only that 30GB volume, which existed for 70% of the month and was then deleted, your final bill will be based on approx 70% of 30GB/month, which is approx 21GB/month. This is under the free tier cap so you shouldn't be charged for it.

Even though you have deleted the volume, the charges accrued by the volume from 1st of the month until today will form a line item on the invoice you receive at the end of the month.

You should be aware of that EBS volume, the moment you have provision a 30GB EBS volume, it will be considered as used space. It does not matter, how much actual data is stored. This is because the whole 30GB EBS volume gets created and attached to EC2. If you don't have any additional need and you don't have to provision additional storage.