I have bought an imac M3. It has thunderbolt 4 USB port (socket) which does not fit my old USB. Can I buy a simple adapter with male thunderbird 4 connector and female USB socket for old USB's
Yes for about $3 or less. Or consider a hub for about $30-60 to turn one port into many ports if you can anticipate ever wanting to connect more than the one thing. Both are abundantly available on Amazon and similar sites.