Hi Hicham,
1. The "Export route control subnet" flag is used for an outbound route control on an L3Out - that is, it ultimately configures an outbound route-map on the L3Out to control the outbound route advertisements.
It doesn't configure a transit routing on its own, but in your scenario, some sort of export route control (done in one way or another) is required as BGP L3Out has export route control hard-enabled.
In addition to this, you need to configure a contract b/w the "server's" L3Out EPG and BGP L3Out EPG to both allow the traffic to pass and the prefixes to be exchanged b/w the two L3Outs.
As you need this contract, your L3Out EPGs should be configured appropriately (i.e., have subnets with External Subnet for External EPG flag configured - I assume yours is an intra-VRF use case).
The static route is automatically redistributed into BGP IPv4 Unicast when it's configured, so there's not need to perform any additional steps for this. However, you can control this redistribution using a Route profile, if so desired.
2. To enable transit routing b/w you always need a contract. However, the route control can be achieved in different ways.
Check out the L3out guide - it's very good and covers what you need to know:
https://www.cisco.com/c/en/us/solutions/collateral/data-center-virtualization/application-centric-infrastructure/guide-c07-743150.html#L3OutTransitRouting
